Metal-air battery components and methods for making same

ABSTRACT

Energy devices such as batteries and methods for fabricating the energy devices. The devices are small, thin and lightweight, yet provide sufficient power for many handheld electronics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.09/589,710, filed on Jun. 8, 2000 now U.S. Pat. No. 6,753,108, which isa continuation-in-part application of U.S. patent application Ser. No.09/532,917 filed on Mar. 22, 2000 now U.S. Pat. No. 6,660,680, which isa continuation-in-part application of U.S. patent application Ser. No.09/141,397, filed on Aug. 27, 1998, now U.S. Pat. No. 6,103,393, whichin turn is a continuation-in-part application of U.S. patent applicationSer. No. 09/028,029, now abandoned, Ser. No. 09/028,277, now U.S. Pat.No. 6,277,169 and Ser. No. 09/030,057, now U.S. Pat. No. 6,338,809, eachfiled on Feb. 24, 1998.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH

This invention was made with United States Government support underCooperative Agreement No. 70NANB8H4045 awarded by the National Institutefor Standards and Technology (NIST).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to energy devices such as batteries andfuel cells and also relates to methods for the fabrication of suchdevices. Specifically, the present invention is directed to energydevices having a reduced thickness that can be fabricated usingtraditional or non-traditional methods to form thin layers within thedevice.

2. Description of Related Art

With the advent of portable and hand-held electronic devices and anincreasing demand for electric automobiles due to the increased strainon non-renewable natural resources, there is a need for the rapiddevelopment of high performance, economical power systems, for examplebatteries and fuel cells, that have a reduced size and weight.

The further size reduction of portable electronic devices is limited bythe inability to provide sufficient power without adding substantialbulk to the device. For example, most of the volume and weight of atypical cellular telephone resides not in the telephone electronics butin the battery required to power the telephone. Small computing devicessuch as laptop computers and personal digital assistants (PDA's) wouldalso benefit from smaller, lighter batteries. Other potential uses forsmall, lightweight batteries include global positioning system (GPS)transceivers.

Batteries can be divided into primary (non-rechargeable) and secondary(rechargeable) batteries. Common types of primary batteries includemetal-air batteries such as Zn-air, Li-air and Al-air, alkalinebatteries and lithium batteries. Common types of secondary batteriesinclude nickel-cadmium, nickel metal hydride and lithium ion batteries.

One type of metal-air battery which offers many competitive advantagesis the zinc-air battery, which relies upon the redox couples of oxygenand zinc to produce energy. Zinc-air batteries operate by adsorbingoxygen from the surrounding air and reducing the oxygen using an oxygenreduction catalyst at the air electrode (cathode). As the oxygen isreduced, zinc metal is oxidized. The reactions of a zinc-air alkalinebattery during discharge are:Cathode: O₂+2H₂O+4e ⁻→4OH⁻Anode: 2Zn→2Zn²⁺+4e ⁻Overall: 2Zn+O₂+2H₂O→2Zn(OH)₂

Typically, air electrodes are alternatively stacked with zinc electrodesand are packaged in a container that is open to the air. When thebattery cell discharges, oxygen is reduced to O²⁻ at the cathode whilezinc metal is oxidized to Zn²⁺ at the anode. Since Zn can beelectrodeposited from aqueous electrolytes to replenish the anode,zinc-air batteries can be secondary batteries as well as primarybatteries.

Among the advantages of secondary zinc-air batteries over otherrechargeable battery systems are safety, long run time and light weight.The batteries contain no toxic materials and operate at one atmosphereof pressure. They can operate as long as 10 to 14 hours, compared to 2to 4 hours for most rechargeable lithium-ion batteries and can be storedfor long periods of time without losing their charge. The light weightof zinc-air batteries leads to good power density (power per unit ofweight or volume), which is ideal for portable applications.

The two major problems associated with secondary zinc-air batteries,however, are limited total power and poor rechargeability/cyclelifetime. Increased power is becoming a major area of attention forbattery manufacturers trying to meet the increased demands of modernelectronics. Current zinc-air batteries can deliver from about 200 to450 W/kg which may enable the batteries to be used in certain low-powerlaptops and other portable devices that have relatively low powerrequirements. Most laptops and other portable electronic devices,however, require batteries that are able to provide a level of powerthat is higher than the capabilities of current zinc-air batteries. Themain reason for the low power of zinc-air batteries is believed to berelated to the inefficiency of the catalytic reaction to reduce oxygenin the air electrodes. Poor accessibility of the catalyst and the localmicrostructural environment around the catalyst and adjoining carbonreduces the efficiency of the oxygen reduction. See, for example, P. N.Ross et al., Journal of the Electrochemical Society, Vol. 131, pg. 1742(1984).

Rechargeability is also a problem with zinc-air batteries. The batterieshave a short cycle life, degrading significantly in performance afterabout 200 recharging cycles or less. The short cycle life of zinc-airbatteries is also believed to be related to the catalyst used in the airelectrodes. Specifically, it is believed that corrosion of the carbonused for the electrocatalyst in these systems leads to a loss incapacity and a decreased discharge time.

Primary (non-rechargeable) alkaline zinc-air batteries are currentlyused to power hearing aids and other devices that require low currentdensities over long periods of time. Zinc-air hearing aid batteries alsoinclude an air cathode and a zinc-based anode. The electrocatalystpowder is formed into a layer for the air cathode which catalyticallyconverts oxygen in the air into hydroxide ion. The hydroxide ion is thentransported in an alkaline electrolyte through a separator to the anodewhere it reacts with zinc to form zincate ion (Zn(OH)₄ ²⁻) and zinc ion(Zn²⁺) and liberates electrons. Improved electrocatalytic layers at theair cathode would advantageously extend the life of such primarybatteries.

In addition to improvements in energy storage, there is a need forimprovements in environmentally friendly and economical energyproduction. Fuel cells are electrochemical devices which are capable ofconverting the energy of a chemical reaction into electrical energywithout combustion and with virtually no pollution. Fuel cells areunlike batteries because fuel cells convert chemical energy toelectrical energy as the chemical reactants are continuously deliveredto the fuel cell. When the fuel cell is off, it has zero electricalpotential. As a result, fuel cells are typically used to produce acontinuous source of electrical energy and compete with other forms ofcontinuous electrical energy production such as the combustion engine,nuclear power stations and coal-fired power stations. The differenttypes of fuel cells are categorized by the electrolyte used in the fuelcell. The five main types of fuel cells are alkaline, molten carbonate,phosphoric acid, solid oxide and proton exchange membrane (PEM) or solidpolymer.

One of the critical requirements for these energy devices is theefficient catalytic conversion of the reactants and a significantobstacle to the wide-scale commercialization of such devices is the needfor highly efficient electrocatalytic layers for this conversionprocess.

One example of a fuel cell utilizing electrocatalytic layers for thechemical reactions is a PEM fuel cell. A PEM fuel cell stack includeshundreds of membrane electrode assemblies (MEA's) each including acathode and anode constructed from, for example, carbon cloth. The anodeand cathode sandwich a proton exchange membrane which has a catalystlayer on each side of the membrane. Power is generated when hydrogen isfed into the anode and oxygen (air) is fed into the cathode. In areaction typically catalyzed by a platinum-based catalyst in thecatalyst layer of the anode, the hydrogen ionizes to form protons andelectrons. The protons are transported through the proton exchangemembrane to a catalyst layer on the opposite side of the membrane (thecathode) where another catalyst, typically platinum or a platinum alloy,catalyzes the reaction of the protons with oxygen to form water. Thereactions can be written as follows:Anode: 2H₂→4H⁺+4e ⁻Cathode: 4H⁺+4e ⁻+O₂→2H₂OOverall: 2H₂+O₂→2H₂OThe electrons formed at the anode are routed to the cathode through anelectrical circuit which provides the electrical power.

The critical issues that must be addressed for the successfulcommercialization of fuel cells are cell cost, cell performance andoperating lifetime. With respect to cell performance, improved powerdensity is critical for automotive applications whereas higher voltageefficiencies are necessary for stationary applications. In terms of fuelcell cost, current fuel cell stacks employ MEA's that include platinumelectrocatalysts with a loading of about 4 milligrams of platinum persquare centimeter on each of the anode and cathode. At a typical cellperformance of 0.42 watts per square centimeter, about 19 grams ofplatinum per kilowatt is required (8 mg Pt per cm² divided by 0.42 wattsper cm²). Platinum metal is very expensive and a significant costreduction in the electrocatalyst is necessary for these cells to becomeeconomically viable. However, reducing the amount of precious metal isnot a suitable solution because there is also a strong demand forimproved cell performance which relies on the presence of a sufficientamount of platinum electrocatalyst.

The major technical challenge is improving the performance of thecathode with air as the oxidant. Platinum metal electrocatalysts foroxygen reduction are used in both alkaline and acid electrolyte mediaand are used in many types of fuel cells including PEM fuel cells,alkaline fuel cells and hybrid fuel cells.

The miniaturization of such devices while providing sufficientelectrical energy to power electronic devices has only been moderatelysuccessful. Among the reasons for the difficulty in fabricating suchdevices is the lack of adequate materials and a manufacturing method forforming very thin layers having good electrical properties includinghigh electrocatalytic activity.

The conventional synthesis of electrocatalyst powders that include anactive species on a support material involves several steps. First, anappropriate high surface area catalyst support (e.g., alumina, titania,silica or carbon) is impregnated with a solution containing a precursorto the active species. Sufficient contact time is maintained for theadsorption of the active species precursor to occur and to achieve auniform deposition of the precursor on the support surface. The catalystis then dried to remove the solvent, for example at temperatures of 100°C. to 120° C. for about 2 to 12 hours. The catalyst is then heated toelevated temperatures, typically 400° C. to 600° C. in air, so that theprecursor is converted to the active species. Typically, oxide catalystsdo not require further treatment.

The foregoing method generally results in poor control over thecomposition and microstructure of the composite electrocatalyst powders.The morphology and surface area of the electrocatalyst powders arecharacteristics that have a critical impact on the performance of thecatalyst. The morphology determines the packing density and the surfacearea determines the type and number of surface adsorption centers wherethe active species are formed during synthesis of the electrocatalyst.The inability to control the fundamental electrocatalyst powdercharacteristics is a major obstacle for the future development ofimproved energy storage and energy production devices, particularlythose having a reduced size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a two-fluid nozzle that is useful for the productionof powders according to the present invention.

FIG. 2 illustrates a spray dryer that is useful for the production ofpowder according to the present invention.

FIG. 3 is a process block diagram showing one embodiment of the processof the present invention.

FIG. 4 is a side view in cross section of one embodiment of aerosolgenerator of the present invention.

FIG. 5 is a top view of a transducer mounting plate showing a 49transducer array for use in an aerosol generator of the presentinvention.

FIG. 6 is a top view of a transducer mounting plate for a 400 transducerarray for use in an ultrasonic generator of the present invention.

FIG. 7 is a side view of the transducer mounting plate shown in FIG. 6.

FIG. 8 is a partial side view showing the profile of a single transducermounting receptacle of the transducer mounting plate shown in FIG. 6.

FIG. 9 is a partial side view in cross-section showing an alternativeembodiment for mounting an ultrasonic transducer.

FIG. 10 is a top view of a bottom retaining plate for retaining aseparator for use in an aerosol generator of the present invention.

FIG. 11 is a top view of a liquid feed box having a bottom retainingplate to assist in retaining a separator for use in an aerosol generatorof the present invention.

FIG. 12 is a side view of the liquid feed box shown in FIG. 11.

FIG. 13 is a side view of a gas tube for delivering gas within anaerosol generator of the present invention.

FIG. 14 shows a partial top view of gas tubes positioned in a liquidfeed box for distributing gas relative to ultrasonic transducerpositions for use in an aerosol generator of the present invention.

FIG. 15 shows one embodiment for a gas distribution configuration forthe aerosol generator of the present invention.

FIG. 16 shows another embodiment for a gas distribution configurationfor the aerosol generator of the present invention.

FIG. 17 is a top view of one embodiment of a gas distribution plate/gastube assembly of the aerosol generator of the present invention.

FIG. 18 is a side view of one embodiment of the gas distributionplate/gas tube assembly shown in FIG. 17.

FIG. 19 shows one embodiment for orienting a transducer in the aerosolgenerator of the present invention.

FIG. 20 is a top view of a gas manifold for distributing gas within anaerosol generator of the present invention.

FIG. 21 is a side view of the gas manifold shown in FIG. 20.

FIG. 22 is a top view of a generator lid of a hood design for use in anaerosol generator of the present invention.

FIG. 23 is a side view of the generator lid shown in FIG. 22.

FIG. 24 is a process block diagram of one embodiment of the process ofthe present invention including a droplet classifier.

FIG. 25 is a top view in cross section of an impactor of the presentinvention for use in classifying an aerosol.

FIG. 26 is a front view of a flow control plate of the impactor shown inFIG. 25.

FIG. 27 is a front view of a mounting plate of the impactor shown inFIG. 25.

FIG. 28 is a front view of an impactor plate assembly of the impactorshown in FIG. 25.

FIG. 29 is a side view of the impactor plate assembly shown in FIG. 28.

FIG. 30 is a process block diagram of one embodiment of the presentinvention including a particle cooler.

FIG. 31 is a top view of a gas quench cooler of the present invention.

FIG. 32 is an end view of the gas quench cooler shown in FIG. 31.

FIG. 33 is a side view of a perforated conduit of the quench coolershown in FIG. 31.

FIG. 34 is a side view showing one embodiment of a gas quench cooler ofthe present invention connected with a cyclone.

FIG. 35 is a block diagram of one embodiment of the present inventionincluding a particle modifier.

FIG. 36 illustrates an SEM photomicrograph of TEFLON-coated carbonparticles according to an embodiment of the present invention.

FIG. 37 illustrates an SEM photomicrograph of electrocatalyst particlesaccording to an embodiment of the present invention.

FIG. 38( a) and (b) illustrate direct-write deposition methods accordingto the present invention.

FIG. 39 illustrates the 3-phase boundary an air cathode of a metal/airbattery.

FIG. 40 illustrates an air cathode according to an embodiment of thepresent invention.

FIG. 41 illustrates a photomicrograph of a current collector accordingto an embodiment of the present invention.

FIG. 42 illustrates an air cathode according to another embodiment ofthe present invention.

FIG. 43 illustrates an air cathode according to another embodiment ofthe present invention.

FIG. 44 illustrates an air cathode according to another embodiment ofthe present invention.

FIG. 45 illustrates an air cathode according to another embodiment ofthe present invention.

FIG. 46 illustrates an air cathode according to an embodiment of thepresent invention including a plurality of monolayers constituting theelectrode.

FIG. 47( a) and (b) illustrate the incorporation of a carbon dioxidereduction layer into an air cathode according to the present invention.

FIG. 48( a) and (b) illustrate a zinc-air battery according to anembodiment of the present invention.

FIG. 49 illustrates a metal/air battery according to an embodiment ofthe present invention.

FIG. 50 illustrates a metal/air battery according to an embodiment ofthe present invention.

FIG. 51 illustrates a membrane electrode assembly according to anembodiment of the present invention.

FIG. 52 illustrates an SEM photomicrograph of an ultrasonicallygenerated electrocatalyst powder according to an embodiment of thepresent invention.

FIG. 53 illustrates a TEM photomicrograph of an ultrasonically generatedelectrocatalyst powder according to an embodiment of the presentinvention.

FIG. 54 illustrates the particle size distribution of an ultrasonicallygenerated electrocatalyst powder according to an embodiment of thepresent invention.

FIG. 55 illustrates the particle size distribution of an ultrasonicallygenerated electrocatalyst powder according to an embodiment of thepresent invention.

FIG. 56 illustrates an SEM photomicrograph of a spray driedelectrocatalyst powder according to an embodiment of the presentinvention.

FIG. 57 illustrates the particle size distribution of a spray driedelectrocatalyst powder according to an embodiment of the presentinvention.

FIG. 58 illustrates the dependence of electrocatalytic activity onMnO_(x) cluster size for an electrocatalyst powder according to anembodiment of the present invention.

FIG. 59 illustrates the dependence of electrocatalytic activity on therelative intensity of XPS peaks for an electrocatalyst powder accordingto the present invention.

FIG. 60 illustrates the effect of increasing manganese concentration onsurface area for a high surface area electrocatalyst powder according toan embodiment of the present invention.

FIG. 61 illustrates the effect of increasing manganese concentration onsurface area for a high surface area electrocatalyst powder according toan embodiment of the present invention.

FIG. 62 illustrates the effect of manganese concentration on thedispersion of the active species on a high surface area electrocatalystpowder according to the present invention.

FIG. 63 illustrates the effect of manganese concentration on thedispersion of the active species on a high surface area electrocatalystpowder according to the present invention.

FIG. 64 illustrates the efficiency of a bifunctional electrocatalystpowder according to the present invention when applied for oxygenreduction reaction.

FIG. 65 illustrates the efficiency of a bi-functional electrocatalystpowder according to the present invention when applied for oxygenevolution reaction.

FIG. 66 illustrates the voltaic efficiency of a bi-functionalelectrocatalyst powder according to the present invention when appliedfor both oxygen reduction and oxygen evolution reaction.

FIG. 67 illustrates the effect of cycling a metal hydride-air cellincluding a bi-functional electrocatalyst powder according to thepresent invention.

FIG. 68 illustrates the capacity of a metal hydride-air cell utilizing abi-functional electrocatalyst powder according to the present invention.

FIG. 69 illustrates a TEM photomicrograph of an electrocatalyst powderaccording to an embodiment of the present invention.

FIG. 70 illustrates a TEM photomicrograph of an electrocatalyst powderaccording to an embodiment of the present invention.

FIG. 71 illustrates the effect of spray conversion temperature onsurface area of an electrocatalyst powder according to an embodiment ofthe present invention.

FIG. 72 illustrates the effect of spray conversion temperature onplatinum metal binding energy for an electrocatalyst powder according toan embodiment of the present invention.

FIG. 73 illustrates the effect of spray conversion temperature on metaldispersion for composite electrocatalyst powders according to anembodiment of the present invention.

FIG. 74 illustrates a TEM photomicrograph of a prior art electrocatalystpowder.

FIG. 75 illustrates a TEM photomicrograph of a prior art electrocatalystpowder.

FIG. 76 illustrates a TEM photomicrograph of an electrocatalyst powderaccording to an embodiment of the present invention.

FIG. 77 illustrates a TEM photomicrograph of an electrocatalyst powderaccording to an embodiment of the present invention.

FIG. 78 illustrates a TEM photomicrograph of an electrocatalyst powderaccording to an embodiment of the present invention.

FIG. 79 illustrates a TEM photomicrograph of an electrocatalyst powderaccording to an embodiment of the present invention.

FIG. 80 illustrates a TEM photomicrograph of an electrocatalyst powderaccording to an embodiment of the present invention.

FIG. 81 illustrates a TEM photomicrograph of an electrocatalyst powderaccording to an embodiment of the present invention.

FIG. 82 illustrates the performance of membrane electrode assemblies inaccordance with an embodiment of the present invention.

FIG. 83 illustrates the performance of membrane electrode assemblies inaccordance with an embodiment of the present invention.

FIG. 84 illustrates the performance of membrane electrode assemblies inaccordance with an embodiment of the present invention.

FIG. 85 illustrates the performance of membrane electrode assemblies inaccordance with an embodiment of the present invention.

FIG. 86 illustrates the performance of membrane electrode assemblies inaccordance with an embodiment of the present invention.

FIG. 87 illustrates the performance of membrane electrode assemblies inaccordance with an embodiment of the present invention.

FIG. 88 illustrates the performance of membrane electrode assemblies inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to energy devices such as primarybatteries, secondary batteries and fuel cells that are fabricated usingfine particles that have unique chemical and physical characteristics,resulting in devices having improved efficiency. The fine particles canadvantageously have controlled chemistry, a small size and a sphericalmorphology. The buoyancy of the particles in a liquid suspension canalso be controlled so that the particles can be adequately suspended ina liquid vehicle for deposition using a direct-write tool. In oneembodiment, the energy devices include components such as electrodesthat are finer and/or thinner than those fabricated by conventionalroutes, enabling the formation of devices having a reduced thickness.The electrodes can be fabricated so that the multiple layers typicallyassociated with an electrode are combined into a multi-functional layer,such as a layer having a compositional gradient through the thickness ofthe layer.

The terms fine powder or fine particles, as used herein, encompassessentially any particulate element or compound having the physicalparticle characteristics described hereinbelow. Preferred fine powdersfor the fabrication of primary batteries, secondary batteries and fuelcells include electrocatalyst powders and other electrically usefulpowders, as is discussed below.

Particularly preferred according to the present invention are compositeelectrocatalyst powders. As used herein, composite electrocatalystpowders or particles are those that include within the individualparticles at least a first active species phase, such as a metal or ametal oxide, that is dispersed on a support phase, such as carbon or ametal oxide. The composite powder batches of the present invention arenot mere physical admixtures of compositionally different particles, butare comprised of particles that include both an active species phase anda support phase. The compositions of the particle components can bevaried independently and various combinations of carbon, metals, metalalloys, metal oxides, mixed metal oxides, organometallic compounds andtheir partial pyrolysis products can be produced as may be required fora particular application. Two or more different materials can beutilized as the active species phase. As an example, combinations of Agand MnO_(x) on a carbon support phase can be useful for someelectrocatalytic applications. Other examples of multiple active speciesare mixtures of porphyrins, partially decomposed porphyrins, Co and CoO.Although carbon is a preferred material for the support phase, othermaterials such as metal oxides can also be useful as the support phase.

In one preferred embodiment, the electrocatalyst powder can includecomposite metal-carbon electrocatalyst particles. Composite metal-carbonelectrocatalyst particles include an active species of at least a firstmetallic phase dispersed on a carbon support phase. The metallic phasecan include any metal and the particularly preferred metal will dependupon the application of the powder. The metallic phase can be metalalloy wherein a first metal is alloyed with one or more alloyingelements. As used herein, the term metal alloy includes intermetalliccompounds between two or more metals. The composite metal-carbonelectrocatalyst powders can also include two or more metals dispersed onthe carbon support as separate phases.

Preferred metals for the electrocatalytically active species include theplatinum group metals and noble metals, particularly Pt, Ag, Pd, Ru, Osand their alloys. The active species phase can also include a metalselected from the group consisting of Ni, Rh, Ir, Co, Cr, Mo, W, V, Nb,Al, Ta, Ti, Zr, Hf, Zn, Fe, Cu, Ga, In, Si, Ge, Sn, Y, lanthanide metalsand combinations or alloys of these metals. Preferred metal alloys foruse according to the present invention include alloys of Pt with othermetals, such as Ru, Os, Cr, Ni, Mn and Co. Particularly preferred amongthese is Pt/Ru for use in hydrogen anodes and Pt/Cr/Co for use in oxygencathodes.

Another preferred embodiment of the present invention is directed tometal oxide-carbon composites that include an active metal oxide speciesdispersed on a carbon support phase. The metal oxide active speciesphase can be selected from the oxides of transition metals, preferablythose existing in oxides of variable oxidation states, and preferablyfrom those having an oxygen deficiency in their crystalline structure.

For example, the dispersed metal oxide can be an oxide of a metalselected from the group consisting of Au, Ag, Pt, Pd, Ni, Co, Rh, Ru,Fe, Mn, Cr, Mo, Re, W, Ta, Nb, V, Hf, Zr, Ti and Al. A particularlypreferred metal oxide according to the present invention is manganeseoxide (MnO_(x), where x has a value from 1 to 2). The dispersed metaloxide active phase can include a mixture of different metal oxides,solid solutions of two or more different metal oxides or double oxides.The metal oxides can be stoichiometric or non-stoichiometric and can bea mixture of one metal oxide having different oxidation states. Themetal oxides can also be amorphous.

For some applications such as secondary metal-air batteries, examples ofelectrocatalyst particles that can be used to catalyze the reduction andoxidation reactions according to the present invention include oxygendeficient metal oxides and metal oxides capable of undergoingreduction/oxidation reactions due to variations in the oxidation statesof the metals contained in the metal oxide. Some compounds aremulti-functional, providing numerous attributes in one compound. Suchoxides do not necessarily have to be dispersed on a support phase.

For example, perovskite phase oxides can be used for electrocatalystswherein the oxides provide electrocatalytic activity, a high surfacearea and electrical conductivity. Specific examples of such perovskitephase oxides include La_(1−x)Sr_(x)Fe_(0.6)CO_(0.4)O₃ (where x is from 0to 1) and La_(1−x)Ca_(x)CoO₃ (where x is from 0 to 1). One particularlypreferred metal oxide electrocatalyst according to the present inventionis an oxygen-deficient cobalt-nickel oxide, Co—Ni—O, which is useful forelectrodes in metal hydride batteries. Although the crystalline phase isnominally NiCO₂O₄, the crystalline phase and hence the electrocatalyticactivity can be well controlled. Other metal oxides within this categoryinclude spinels of the general formula AB₂O₄ where A is selected from adivalent metal such as Mg, Ca, Sr, Ba, Fe, Ru, Co, Ni, Cu, Pd, Pt, Eu,Sm, Sn, Zn, Cd, Hg or combinations thereof and B is selected fromtrivalent metals such as Co, Mn, Re, Al, Ga, In, Fe, Ru, Os, Cr, Mo, W,Y, Sc, lanthanide metals or combinations thereof. Other useful metaloxides include the oxides of Ni, Co, Fe, Ti, Zr, Zn, In, In—Sn, Ga, Ru,Cr, Mo, W, Cu, Va, Ng and Ta metal gallates, metal ruthenates, andcopper containing perovskite phase metal oxides.

A further class of catalysts that can be useful according to the presentinvention are those derived from molecular compounds that are eitherdispersed on a support phase or that have no support phase. Examples ofsuch materials are metal porphyrin complexes which catalyze thereduction of O₂ to OH⁻ but are oxidized during the oxidation of OH⁻.These species are suitable for primary batteries and fuel cells such asalkaline fuel cells. Included in this group are metal porphyrincomplexes of Co, Fe, Zn, Ni, Cu, Pd, Pt, Sn, Mo, Mn, Os, Ir and Ru.Other metal ligand complexes can be active in these catalytic oxidationand reduction reactions and can be formed by the methods describedherein. Such metal ligands can be selected from the class of N4-metalchelates, represented by porphyrins, tetraazaanulens, phtalocyanines andother chelating agents. In some cases the organic ligands are active incatalyzing reduction and oxidation reactions and in some cases theligands are active when they remain intact, as might be the case for anintact porphyrin ring system, or they might be partially reacted duringthermal processing to form a different species that could also be activein the catalytic reactions. An example is the reaction product derivedfrom porphyrins or other organic compounds.

Carbon is required for the reduction of O₂ to OH⁻ and is believed to beinvolved in the reduction of peroxide to hydroxide ion. Othercarbon-based active species include homo- and hetero-fullerene andcarbon nanotube based materials.

With respect to the composite electrocatalyst particles, thecharacteristics of the secondary support phase, the primary particlesconstituting the support phase and the active species can beindependently controlled to yield different performance characteristicsfor a particular application.

Many primary and secondary batteries utilize lithium-based fine powdersfor an electrode material and the present invention is also applicableto fine powders of LiCoO₂, LiNiO₂ and LiMn₂O₄, as well as otherlithium-based compounds.

Powders of metals and metal alloys (including intermetallic compounds)are also useful for electrodes, particularly anodes in certain batterycells. Examples of such metal compounds include LaNi₅, La—Ni—Co—Al,Nd—Ce—Ni—Co—Al and V—Ti—Zr—Ni.

In addition to the electrocatalytic powders, other fine powders areuseful for fabricating energy device components according to the presentinvention. Among these are the supporting materials, hydrophobicmaterials, electroconductive materials and insulator materials such asdielectrics for separating membranes. For example, metals such as silver(Ag) and nickel (Ni) are useful for the current collectors in batterycells.

According to one embodiment of the present invention, carbon particlesare “Teflonized” by coating the carbon particles with atetrafluoroethylene (TFE) fluorocarbon polymer such as TEFLON (E.I. duPont de Nemours, Wilmington, Del.). These Teflonized carbon particlescan be used to form hydrophobic layers in an energy device, as isdiscussed below. The hydrophobicity can be controlled by controlling theratio of TEFLON to carbon. For some applications, TEFLON can also bedeposited on electrocatalyst particles.

As is discussed above, the composite electrocatalyst powders include asecondary support phase typically consisting of agglomerates of smallerprimary particles such as carbon or a metal oxide which supports theactive species. Two or more types of primary particles can be mixed toform the secondary support phase. As an example, two or more types ofparticulate carbon (e.g., amorphous and graphitic) can be combined toform the secondary support phase. The two types of particulate carboncan have different performance characteristics that combine to enhancethe performance of the catalyst.

It is an advantage of the present invention that the composition of theelectrocatalyst particles can be homogeneous. A degree of homogeneity inmaterials is often not obtainable by traditional forming methods such asliquid precipitation. However, it is also possible according to thepresent invention to provide compositional gradients within theelectrocatalyst particles. For example, the active species concentrationin a composite particle can be higher or lower at the surface of thesupport phase than near the center and gradients corresponding tocompositional changes of 10 to 100 weight percent can be obtained. Whenthe particles are deposited by direct-write deposition, the particlesretain their structural morphology and therefore the functionality ofthe compositional gradient can be exploited.

The preferred form of carbon for crystalline supported materials arethose which are amorphous. The preferred carbons for supported metalslike Pt are carbons that are crystalline since Pt dispersion is favoredby reduced carbon surfaces since such surfaces have substantially nosurface hydroxyls. For supported MnO_(x), it is preferred to have acrystalline carbon support. Preferably, the crystallinity of the primaryparticles constituting the support phase is controlled through theproper selection of precursor materials. Graphitic carbon is preferredfor long term operational stability of fuel cells and batteries.Amorphous carbon is preferred when a smaller crystallite size is desiredfor the supported active species.

The overall density of the secondary support phase is related to theporosity of the support phase. It is preferred that the accessible(e.g., open) porosity in the composite electrocatalyst particles is atleast about 5 percent and more preferably is at least about 40 percent.The pore size and size distribution in the secondary support phase canalso be controlled and the average pore size is preferably from about 10to 100 nanometers. High porosity is advantageous for rapid transport ofspecies into and out of the secondary structures. Lower particledensities also permit the particles to be suspended in a liquid vehiclefor use in printing techniques such as ink jet deposition wheresuspension of particles for long periods is required. As an example, anaerogel carbon or metal oxide can have a density much lower than 1g/cm³.

Agglomeration of the electrocatalyst particles can affect the propertiesof the particles such as dispersion into liquids used to deposit theparticles. It is therefore preferred that minimal agglomeration of theparticles exist in the powder batch.

It is also an advantage of the present invention that the fine particlesare substantially spherical in shape. That is, the particles arepreferably not jagged or irregular in shape. Spherical particles canadvantageously be deposited using a variety of techniques, includingdirect-write deposition, and can form smooth, thin layers that have ahigh packing density.

In addition, the composite electrocatalyst powders according to thepresent invention have a surface area of at least about 25 m²/g, morepreferably at least about 90 m²/g and even more preferably at leastabout 600 m²/g. Surface area is typically measured using the BETnitrogen adsorption method which is indicative of the surface area ofthe powder, including the surface area of accessible pores on thesurface of the particles. High surface area generally leads to increasedcatalytic activity.

The composite electrocatalyst particles preferably include a carbonsupport with at least about 10 weight percent and more preferably atleast about 20 weight percent of the catalytically active speciesdispersed on the support surface. The particles preferably include notgreater than about 90 weight percent, more preferably not greater thanabout 80 weight percent carbon. In one embodiment, the compositeparticles include from about 20 to about 40 weight percent of the activespecies phase. It has been found that such compositional levels giverise to the most advantageous electrocatalyst properties. However, thepreferred level of the active species dispersed on the carbon supportwill depend upon the total surface area of the carbon, the type ofactive species and the application of the powder. A carbon supporthaving a very high surface area will accommodate a higher percentage ofmetal on its surface and is therefore preferred.

It is preferred that the average size of the active species phasedispersed on the carbon support is such that the particles include smallsingle crystals or crystallite clusters, collectively referred to hereinas clusters. Accordingly, the average active species cluster size ispreferably not greater than about 20 nanometers, more preferably is notgreater than about 10 nanometers and even more preferably is not greaterthan about 5 nanometers, such as from about 0.5 to 5 nanometers.According to one embodiment of the present invention, at least about 20weight percent and more preferably at least about 30 weight percent ofthe active species clusters have a size of not greater than about 3nanometers. Composite electrocatalyst powders having such smallcrystallite clusters advantageously have enhanced catalytic propertiesas compared to composite powders comprising an active species phasehaving larger crystallites. The powder production method of the presentinvention advantageously permits control over the crystallinity bycontrolling the reaction temperature and/or residence time, as isdiscussed below.

When the active species includes a metal, the oxidation state of themetal in the metal phase is preferably close to zero, i.e., a puremetal. It is believed that higher oxidation states are detrimental toelectrocatalyst powder activity. The powder production method of thepresent invention advantageously enables good control over the oxidationstate of the metal.

The fine powders of the present invention have a well-controlledparticle size. Preferably, the volume average particle size is notgreater than about 100 μm, more preferably not greater than about 20 μm,even more preferably not greater than about 10 μm and even morepreferably not greater than about 5 μm. Further, it is preferred thatthe volume average particle size is at least about 0.3 μm, morepreferably at least about 0.5 μm and even more preferably at least about1 μm. As used herein, the average particle size is the median particlesize (d₅₀). Powder batches having an average particle size within thepreferred parameters disclosed herein enable the formation of thinelectrocatalytic layers which are advantageous for producing energydevice such as batteries and fuel cells according to the presentinvention.

The size distributions of the secondary support phase, the primaryparticles, and the active species phase are important in determiningperformance and can be controlled using the powder production methodaccording to the present invention. Narrower particle size distributionsare preferred for the secondary support phase to allow printing of theparticles through a narrow orifice without clogging. For example, it ispreferred that at least about 50 weight percent of the particles have asize of not greater than about two times the average particle size andit is more preferred that at least about 75 weight percent of theparticles have a size of not greater than about two times the averageparticle size. The particle size distribution can be bimodal or trimodalwhich can advantageously provide improved packing density in depositedlayers.

The powders produced by the processes described herein, namely spraypyrolysis or spray conversion, can include some agglomerates ofspherical particles. Micrometer-sized particles often form softagglomerates as a result of their relatively high surface energy(compared to larger particles). Such soft agglomerates may be dispersedeasily by treatments such as exposure to ultrasound in a liquid mediumor sieving. The particle size distributions described herein aremeasured by mixing samples of the powders in a medium such as water witha surfactant and a short exposure to ultrasound through either anultrasonic bath or horn. The ultrasonic horn supplies sufficient energyto disperse the soft agglomerates into the primary spherical particles.The primary particle size distribution is then measured on a volumebasis by light scattering, such as in a MICROTRAC particle size analyzer(Honeywell Industrial Automation and Control, Fort Washington, Pa.).This provides a good measure of the useful dispersion characteristics ofthe powder because this simulates the dispersion of the particles in aliquid medium such as a paste or slurry that is used to deposit theparticles in a device. Thus, the particle size referred to herein refersto the particle size after dispersion of soft agglomerates.

The fine powder batches useful according to the present invention arepreferably produced by a process referred to as spray conversion orspray pyrolysis. In this process, a liquid precursor is converted toaerosol form and liquid from the droplets in the aerosol is then removedto permit formation of the desired particles in a dispersed state.Although the powder batch is typically manufactured in a dry state, thepowder may, after manufacture, be placed in a wet environment, such asin a paste or slurry.

The method for the production of the composite electrocatalyst powdersgenerally includes the steps of: providing a liquid precursor whichincludes a precursor to the support phase (e.g., carbon) and a precursorto the active species; atomizing the precursor to form a suspension ofliquid precursor droplets; and removing liquid from liquid precursordroplets to form the powder. For electrocatalysts and other powders thatare not supported, the precursor to the support phase is not necessary.At least one component of the liquid precursor is chemically convertedinto a desired component of the powder. The drying of the precursors andthe conversion to a catalytically active species are advantageouslycombined in one step, where both the removal of the solvent and theconversion of a precursor to the active species occur essentiallysimultaneously. Combined with a short reaction time, this enablescontrol over the distribution of the active species on the support, theoxidation state of the active species and the crystallinity of theactive species. By varying reaction time, temperature, type of supportmaterial and type of precursors, the method of the present invention canproduce catalyst morphologies and active species structures which yieldsignificantly improved catalytic performance.

A unique characteristic of the present process is the simultaneousformation of a secondary support phase such as carbon. The secondarysupport phase forms as a result of the formation and drying of thedroplets during the spray conversion process and the characteristics ofthe primary support particles such as particle size, particle sizedistribution and surface area influence the properties of the supportphase.

The first step in the fabrication of the electrocatalyst particlesaccording to the present invention is to form a liquid precursor to theparticles. In the case of composite electrocatalyst powders, the liquidprecursor includes a precursor to both the active species phase and thesupport phase. Proper selection of the precursors enables the productionof particles having well-controlled chemical and physical properties.

For the production of metal-carbon composite electrocatalyst particlesthe precursor solution includes at least one metal precursor. The metalprecursor may be a substance in either a liquid or solid phase.Preferably, the metal precursor will be a metal-containing compound,such as a salt, dissolved in a liquid solvent of the liquid feed. Forexample, the precursor solution can include nitrates, chlorides,sulfates, hydroxides, or carboxylates of a metal. In this case, themetal precursor will undergo one or more chemical reactions in thefurnace to convert to a metallic state and form the electrocatalystparticles. It may be desirable to acidify the precursor solution toincrease the solubility, such as by adding hydrochloric acid.

A preferred catalytically active metal according to the presentinvention is platinum (Pt) and preferred precursors for platinum metalaccording to the present invention are chloroplatinic acid(H₂PtCl₆·xH₂O), Pt(NH₃)₄(NO₃)₂ and hydroxoplatinic acid (H₂Pt(OH)₆).Other platinum precursors include Pt-nitrates, Pt-ammine nitrates,PtCl₄, Na₂PtCl₄, and the like. Chloroplatinic acid is soluble in waterand the solutions advantageously maintain a low viscosity.Hydroxoplatinic acid is advantageous since it converts to platinum metalat relatively low temperatures.

Palladium precursors can include inorganic Pd salts such as H₂PdCl₄ orNa₂PdCl₄, complex Pd salts such as Pd(NH₃)₄Cl₂ or Pd(NH₃)₂(OH)₂,Pd-carboxylates, and the like. For silver, inorganic salts can be usedincluding Ag-nitrate ammine complexes, Ag-carboxylates and Ag-oxalate.For osmium, inorganic salts such as OsCl₃ can be used. For copper,inorganic salts such as Cu-carboxylates can be used.

For the production of metal oxide-containing electrocatalyst powders,including supported and unsupported metal oxides, a precursor to themetal oxide must be included in the precursor solution. For metaloxides, including oxides of Au, Ag, Pt, Pd, Ni, Co, Rh, Ru, Fe, Mn, Cr,Mo, Re, W, Ta, Nb, V, Hf, Zr, Ti and Al, inorganic salts includingnitrates, chlorides, hydroxides, halides, sulfates, phosphates,carboxylates, oxylates and carbonates can be used as precursors.Particulate oxides of the metals can also be used as a precursor to ametal oxide in the final powder.

Particularly preferred metal oxide precursors include: K₂Cr₂O₇,Cr-carboxylates and chromium oxalate for chrome oxide; KMnO₄,Mn-nitrate, Mn-acetate, Mn-carboxylates, Mn-alkoxides and MnO₂ formanganese oxide; Na₂WO₄ and W₂O₃, for tungsten oxide; K₂MoO₄ and MoO₂for molybdenum oxide; Co-ammine complexes, Co-carboxylates and cobaltoxides for cobalt oxide; Ni-ammine complexes, Ni-carboxylates and nickeloxide for nickel oxide; and Cu-ammine complexes, Cu-carboxylates andcopper oxides for copper oxide.

For the production of composite powders having a carbon support phase,the precursor solution also includes at least one carbon precursor. Thecarbon precursor can be an organic precursor such as carboxylic acid,benzoic acid, polycarboxylic acids such as terephthalic, isophthalic,trimesic and trimellitic acids, or polynuclear carboxylic acids such asnapthoic acid, or polynuclear polycarboxylic acids. Organic precursorscan react by a mechanism such as:aM(NO₃)_(n)+b(C_(x)H_(y)O_(z))_(m)→M_(a)C_(b)

The use of a liquid organic carbon precursor typically results inamorphous carbon, which is not desirable for most electrocatalystapplications. Therefore, the carbon support precursor is preferably adispersion of suspended carbon particles. The carbon particles can besuspended in water with additives, such as surfactants, to stabilize thesuspension. The carbon particles used as the precursor are the primaryparticles which constitute the secondary support phase.

The carbon precursor particles preferably have a BET surface area of atleast about 60 m²/g, more preferably at least about 250 m²/g, even morepreferably at least about 600 m²/g and most preferably at least about1300 m²/g. The surface area of the particulate carbon precursor stronglyinfluences the surface area of the composite electrocatalyst powder, andtherefore strongly influences the electrocatalytic activity of thecomposite powder.

The particulate carbon is small enough to be dispersed and suspended inthe droplets generated from the liquid precursor. Therefore, theparticulate carbon preferably has an average size of from about 10 toabout 100 nanometers, more preferably from about 20 to about 40nanometers. The carbon can be crystalline (graphitic), amorphous or acombination of different carbon types. The particles can also have agraphitic core with an amorphous surface or an amorphous core with agraphitic surface.

The surface characteristics of the primary particles making up thesecondary support structures (e.g., carbon) can be varied. It ispreferred that the surface of the primary particles allow adequatedispersion of the precursor particles into the precursor liquid. Afterprocessing to form the secondary structures, it is preferred that thesurfaces have a controlled surface chemistry. Oxidized carbon surfacescan expose hydroxyl, carboxyl, aldehyde, and other functional groupswhich make the surface more hydrophilic. Reduced carbon surfacesterminate in hydrogen which promotes hydrophobicity. The ability tocontrol the surface chemistry allows tailoring of the hydrophobicity ofthe surfaces, which in turn allows the formation of gradients inhydrophobicity within beds of deposited particles. Oxidized carbonsurfaces also tend to be microetched, corresponding to higher surfaceareas while reduced carbon surfaces have lower surface areas. Oxidizedcarbon surfaces can be derivatized by reaction with various agents whichallows coupling of various oxygen containing groups with the surface tofurther tailor the surface chemistry. This advantageously enables theaddition of inorganic, organic, metal organic or organometalliccompounds to the surface. As is discussed above, a TEFLON emulsion canbe used in the precursor to coat the carbon particles with a TEFLONlayer, thereby controlling the hydrophobicity of the particles.

Among the convenient sources of dispersed carbon are commerciallyavailable carbon-based lubricants which are a suspension of fine carbonparticles in an aqueous medium such as dispersed carbon black.Particularly preferred are acetylene carbon blacks having high chemicalpurity and good electrical conductivity. Examples of such carbonsuspensions that are available commercially are GRAFO 1300 (FuchsLubricant, Co., Harvey, Ill.) which is a suspension of VULCAN XC-72carbon black (Cabot Corp., Alpharetta, Ga.) having an average size ofabout 30 nanometers and a surface area of about 254 m²/g. Even morepreferred are BLACKPEARLS 2000 (Cabot Corp., Alpharetta, GA) and KETJENBLACK (Akzo Nobel, Ltd., Amersfoort, Netherlands), each of whichincludes carbon having a specific surface area of from about 1300 to1500 m²/g. Another preferred class of carbon materials are activatedcarbons which have a degree of catalytic activity, such as NORIT NK(Cabot Corp., Alpharetta, Ga.).

The precursors can be formed into an aerosol by a number of methods, asis discussed below. When ultrasonic atomization is used for aerosolgeneration, the solids loading of carbon in the precursor solutionshould not be so high as to adversely affect aerosol generation and ispreferably not greater than about 10 weight percent, more preferably notgreater than about 5 weight percent. When a spray nozzle is used foraerosol generation, the solids loading of carbon is preferably notgreater than about 20 weight percent. However, larger spray nozzles withhigh pressure lines can accommodate greater than about 40 weight percentsolids.

It is preferable to mechanically dissociate larger aggregates of thecarbon powders by using, for example, a blade grinder or other type ofhigh-speed blade mill. Thus, dispersing the carbon powder in waterpreferably includes: 1) if not already provided in suspension, wettingof the carbon black powder by mixing a limited amount of the dry powderwith a wetting agent and a soft surfactant; 2) diluting the initialheavy suspension with the remaining water and a basic surfactant dilutedin the water; and 3) breaking secondary agglomerates by sonification ofthe liquid suspension in an ultrasonic bath.

The precursor to the metal or metal oxide active species, for examplepotassium permanganate, is preferably dissolved separately in water andadded in an appropriate amount to a carbon suspension, prior to breakingthe secondary agglomerates. Adding the metal salt in this manneradvantageously facilitates breaking the larger agglomerates and themixing results in a less viscous slurry. After sonification, theslurries are stable for several months without any apparentsedimentation or separation of the components.

Spray dryers are not commonly used for spray conversion whereinprecursors to a material component are dried and reacted in one step.Nanometer-sized particles are difficult to produce in the presence ofother particles while maintaining control of their dispersion on asupport surface. Converting the precursors in a spray drier is possibleaccording to the present invention due to the use of precursors andadditives that decompose at a temperature that is preferably not greaterthan about 400° C., more preferably not greater than about 300° C.

Low thermal decomposition temperature precursors that are useful at suchlow reaction temperatures according to the present invention to formmetals include carboxylates, hydroxides, halides, nitrates,metal-organic complexes, amine adducts, isonitrile compounds, Schiffbase complexes, b-diketonates, alkyls, phosphine complexes, phosphitecomplexes and carbonyl complexes of metals such as Ni, Ag, Pd, Pt, Rh,Sn, Cu, Au, Co, Ir, Ru and Os.

For metal oxides, useful low temperature precursors includeoxocomplexes, alkoxides, amides, carboxylates, hydroxides, halides,nitrates, metal-organic complexes, amine adducts, isonitrile compounds,Schiff base complexes, b-diketonates, alkyls, phosphine complexes,phosphite complexes and carbonyl complexes of metals such as Sc, Y, La,lanthanides, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co,Rh, Ir and Sn.

When a metal is the active species phase, additives to ensure reductionto the metal at a low temperature can advantageously be used and willgenerally be soluble reducing agents and may either reduce the dissolvedcomplex before spraying or during spraying. Preferably, the reducingagent will not substantially reduce the precursor at room temperature,but will cause reduction at an elevated temperature between about 100°C. and 400° C. These reducing agents should also be water stable and anyvolatile species that form from the reduction should be capable of beingremoved from the system. Examples of such reducing agents includeboranes, borane adducts (e.g., trimethylamineborane, BH₃NMe₃), silanederivatives, e.g., SiH_((4−x))R_(x) (where R=an organic group, aryl,alkyl, or functionalized alkyl or aryl group, polyether, alkylcarboxylate) borohydrides, e.g., NaBH₄, NH₄BH₄, MBH_((3−x))R_(x) (whereR=an organic group, aryl, alkyl, or functionalized alkyl or aryl group,polyether, alkyl carboxylate). Other reducing agents include alanes ortin hydrides.

According to a particularly preferred embodiment, a reducing agent forPt metal is selected from the group consisting of formic acid,formaldehyde, hydrazine and hydrazine salts. For example, an acidifiedsolution of H₂Pt(OH)₆ in the presence of formic acid is stable at roomtemperature but is reduced to Pt metal at low reaction temperatures,such as about 100° C.

For a metal oxide as the active species phase, additives to ensureoxidation to the metal oxide at low temperature can also be used andwill generally be soluble oxidizing agents and may either oxidize thedissolved complex before spraying or during spraying. Preferably, theoxidizing agent will not oxidize the precursor to the metal oxide atroom temperature, but will cause reduction at elevated temperaturebetween about 100° C. and 400° C. These species should also be waterstable and form volatile species that can be removed from the system.Examples include amine oxides, e.g., trimethylamine-N-oxide (Me₃NO),oxidizing mineral acids such as nitric acid, sulfuric acid and aquaregia, oxidizing organic acids such as carboxylic acids, phosphineoxides, hydrogen peroxide, ozone or sulfur oxides.

The precursor solution can include other additives such as surfactants,wetting agents, pH adjusters or the like. It is preferred to minimizethe use of such additives, however, while maintaining good dispersion ofthe precursors. Excess surfactants, particularly high molecular weightsurfactants, can remain on the electrocatalyst particle surface anddegrade the catalytic activity if not fully removed.

Spray conversion or spray pyrolysis is a valuable processing methodbecause the particles are raised to a high temperature for a shortperiod of time. The relatively high temperature achieves conversion ofthe molecular precursor to the final desired phase, but the short timeensures little surface diffusion that can cause agglomeration of thenanometer-sized active phase. Hence, the support phase is formed withwell dispersed nanometer sized active phase particles.

The liquid precursor is then atomized to form a suspension of droplets.The droplets in the suspension have a droplet size distribution that isdetermined by the characteristics of the atomizer. Preferred atomizersinclude single-fluid nozzles, two-fluid nozzles, ultrasonic nozzles androtary wheel atomizers. Ultrasonic fountains can also be used. Anexample of an ultrasonic fountain generator and apparatus is disclosedin commonly-owned U.S. patent application Ser. No. 09/030,057, which isincorporated herein by reference in its entirety.

Each of the foregoing aerosol generation methods produces particles witha characteristic size distribution, ejection velocity and spray pattern.The particle size is an important property to consider when selecting anatomization method. Properties such as final particle size, impactionefficiency, settling rate, heating rate and evaporation rate all dependon the precursor droplet size. Other factors to consider when selectingan atomizer include production rate, system orientation, processingtemperature and precursor composition. For example, some precursorcompositions cannot be economically atomized by ultrasonic methods.

Ultrasonic fountains and ultrasonic nozzles produce droplets by creatingcapillary waves on a liquid surface. The droplets are ejected from thebulk liquid with a slow velocity and a carrier gas is required to sweepthe droplets away from the generation zone.

The manner in which the carrier gas is introduced to the system plays animportant part in the droplet capture efficiency for both ultrasonicsystems. For ultrasonic fountains, the carrier gas is introduced byeither a point source close to the fountain or by sweeping the gasaround the perimeter of the cone and removing the gas through a tubeplaced at a predetermined height above the liquid surface. For a pointsource, the dimension of the source hole should be appropriately sizedto ensure that the gas captures the maximum number of particles per unittime. When gas is delivered to several locations via the same tube, theholes must be sized such that the flow rate is divided equally among theholes. In addition, the position of the source relative the height abovethe liquid level and transducer level must be selected such that the gascaptures the maximum number of particles per unit time.

The gas flow rate also determines the number of droplets removed fromthe generation zone and delivered to the reactor per unit time. Anincrease in the carrier-gas flow rate usually results in an increase inthe droplet removal rate. However a threshold gas flow rate existsbeyond which the droplet removal rate reaches an asymptotic value ordeclines due to distortions in the fountain caused by the highervelocity gas. For ultrasonic fountains this optimum flow rate is on theorder of about 5 slpm per transducer when delivered through ⅛″ or 3/32″holes located within ½″ to 1″ of the fountain. When the carrier gas isdelivered concentrically and radially inward around the fountain, thedroplets are entrained in the gas and removed via a “pick-up” tubesituated directly above the fountain. The aerosol-laden stream istransported through the pickup tube and into the reactor. The height ofthe tube above the precursor liquid level must be adjusted along withthe carrier gas flow rate to maximize droplet removal rates per unittime. Preferred tube heights above the liquid surface are between about0.5 and 1.75 inches and preferred gas flow rates are between about 2.5and 10 slpm. The diameter of the tube must be selected relative to thefountain dimensions to ensure adequate removal of the droplets andminimize aspiration and percolation of condensation on the insidediameter of the tube. For 20 mm diameter transducers operating at 1.6MHz, the preferred tube inner diameter is between about 0.5 and 1 inch.For both point source and radial gas delivery, the actual optimum flowrates are material dependent.

Translating the concept of a radially-inward sweeping gas tomulti-transducer configurations can be accomplished by one of twomethods. Both methods incorporate the use of a gas distribution systemand a gas focusing system. The purpose of the gas distribution system isto ensure that the carrier gas is distributed evenly among eachtransducer or gang of transducers. The focusing system focuses thecarrier gas around the fountain(s) such that the droplet removal rate ismaximized. An example of the focusing system is the pickup tubedescribed above. The aerosol-laden carrier gas is transported throughthe pickup tube(s) or zones and the aerosol streams from each pickupzone are combined and transported to the reactor.

In the first method, an array of pickup tubes of optimum diameter can bearranged such that the center of each tube is aligned over an individualfountain (one tube for each fountain). The carrier gas is evenlydistributed among the tubes to ensure each transducer has suitableconditions for droplet removal. The pickup tubes transport the aerosolstreams to a zone where all streams are combined and delivered to thereactor. In this configuration, the pickup tube acts as the focusingdevice.

Another configuration allows groups of transducers to be used togetherin such a way that a large pickup channel or other design can focus gasaround and remove droplets from the group of transducers. An examplewould be to align the transducers in a line and have a rectangularpickup channel positioned above the transducers such that the openinterior of the rectangle is oriented above the group of fountains. Thepickup channel acts as the focusing device and the carrier gas sweepsaxially inward towards the centerline of the rectangle. Again, theheight of the pickup channel above the fountains must be adjusted formaximum particle removal rates. The carrier gas must be evenlydistributed so that each transducer experiences the conditions necessaryfor maximum particle removal rates.

Precursor temperature and transducer temperature have an effect onaerosol generation rate and transducer longevity in ultrasonicfountains. Preferably, the precursor fluid and transducer are maintainedat a temperature of from about 25° C. to about 40° C. The distancebetween the transducer and the precursor liquid surface also influencesthe aerosol generation rate.

Ultrasonic fountains create a large number of droplets that arere-entrained into the bulk precursor fluid. These droplets lose waterquickly and cause the bulk fluid to concentrate over time. This requiresmonitoring and adjustment of the solution to prevent the precursorconcentration from changing over time. Typically, water is the onlyvolatile component and a density measurement can be used to monitor thesolution and make-up water can be used to re-adjust the solutionconcentration. More complex solutions may require monitoring of the pHand the addition of other components such as acids or bases.

Typically, ultrasonic fountains operating in the MHz frequency rangeproduce smaller mean particle sizes with narrower particle sizedistributions compared to ultrasonic nozzles operating in the 100 KHzrange and single- or two-fluid spray nozzles. However, ultrasonicfountains typically generate fewer droplets per unit time per device andare less efficient in terms of energy conversion than other systems.Ultrasonic nozzles, however, can atomize a wider variety of solutionsmore effectively than ultrasonic fountains.

Like ultrasonic fountains, ultrasonic nozzles atomize fluid by creatingcapillary waves on the surface of a liquid. The liquid is delivered tothe vibrating tip where it is atomized, ejected from the surface andcarried away from the surface by a carrier gas or by gravity. Totransport the particles to the reactor, a carrier gas is passed throughthe generation zone at velocities that do not disrupt the atomizationprocess.

Commercially available ultrasonic nozzles have frequencies ranging fromtens of KHz to hundreds of KHz and have a variety of tip configurationsselected according to the desired spray pattern. The lower frequenciesallow for greater power input compared to ultrasonic fountains operatingin the MHz range and thus a larger variety of materials can be atomized.However, lower frequencies produce larger particles than higherfrequencies such that the ultrasonic nozzle is unsuitable forapplications requiring a small particle size. Flow rates for ultrasonicnozzles are typically larger than ultrasonic fountains which make themquite attractive for applications requiring high volume withoutrequiring a small droplet size.

Scale up of ultrasonic nozzles can be accomplished by using a unit ratedfor larger volumetric flow rates or by using multiple units. Largerunits produce particles having similar size distributions as smallerunits, provided the operating frequencies are similar.

Single-fluid and two-fluid nozzles create droplets from liquid jetssheared apart from forces generated at the gas-liquid interface. Whereasboth rely on velocity differentials between the liquid and a gas, thesingle-fluid nozzle uses ambient air and high liquid velocities whilethe two-fluid nozzle uses a gas stream to create high gas velocities andpressures that shear the liquid jet stream into droplets. Because highvelocity differentials are required to accomplish atomization, theresultant particles typically have high kinetic energy compared toultrasonic techniques. Also, the trajectories of the particles candiverge greatly and the outer boundary of these spray patterns can bequite large requiring a larger reactor system in order to minimizeparticle losses on the walls.

The size distributions of particles produced by single- and two-fluidnozzles are typically larger than those produced by ultrasonictechniques. Droplet diameters can be on the order of about 40 μm to 60μm. For horizontally-oriented systems, larger particles will settle outmore quickly than smaller particles and therefore ultrasonic techniquesmay be advantageous for horizontal systems. However, the productionrates of spray nozzles are typically higher and are thus a favorablemeans of atomization, provided that the reactor can reach the necessarytemperatures and the nozzle can generate droplets having a desired sizeand size distribution. Also, spray nozzles can atomize a wider varietyof precursors and at higher precursor concentrations, such as precursorswith a high loading of particulate carbon.

Another advantage of spray nozzles is the simplicity of the design.Unlike ultrasonic methods, spray nozzles do not require electroniccomponents to operate. Compressed gas and a pressurized liquid sourceare the only requirements. The mechanical components have few movingparts, minimizing the likelihood of breakdowns.

A two-fluid nozzle is illustrated in FIG. 1. The two fluid nozzle 100includes a central aperture 102 for directing the liquid precursor intothe chamber. Two outer apertures 104 and 106 direct a jet of air orother gas toward the liquid precursor stream as the liquid precursor issprayed out of the central aperture 102. Atomization is accomplished bylarge shear forces that are generated when the low-velocity liquidstream encounters the high-velocity gas streams. The particle sizecharacteristics of the aerosol are dependent on the flow rate of thegas.

After atomization, water is removed from the liquid droplets byevaporation. Preferably, the aerosol of liquid droplets is passedthrough a heating zone, such as a hot-wall reactor or a spray dryer toremove the liquid.

Hot-wall reactors transfer heat into the particle by maintaining a fixedwall temperature within the reaction zone. The carrier gas absorbs heatfrom the walls of the system until it reaches thermal equilibrium withthe reactor walls. Energy transport must occur from the walls throughthe bulk of the gas, thus some component of convective heat transportwithin the gas is desirable as opposed to purely conductive transport.Horizontal hot-wall reactors have naturally occurring convective cellsthat are the result of buoyant forces generated within the reactor.However, highly turbulent systems may give rise to undesirable particledeposition and losses. Preferably, Reynolds numbers within the systemshould be between 300 and 2400 based on maximum wall temperature,Prandtl numbers should be between 0.6 and 0.8 and Grashof numbers shouldbe between 1×10⁵ and 4×10⁷ based on gas inlet temperature and maximumwall temperature. Residence times within the reactor are preferably theorder of from about 1.5 to 4 seconds.

The advantages of the hot-wall reactor include the ability to controlthe time/temperature history of the particle with greater precision overlonger time intervals and the ability to achieve higher processingtemperatures. The disadvantages of the hot-wall reactor include limitedscalability due to thermal transport properties and equipment costs.

Spray dryers accomplish heat transfer by intimately mixing a hot gasstream with an aerosol stream. The time-temperature history of theparticle is a function of the inlet temperatures of the inlet streams,the compositions of the inlet streams, the relative mass flow rates ofthe streams and the degree of mixing in the drying chamber. The outlettemperature can be calculated from a simple thermodynamic energy balanceequating the energy change of two streams and solving for theequilibrium temperature. A spray dryer has the advantages of highercapacity, lower operating cost, less complex equipment and simplescalability.

Most commercial spray dryers are designed to accommodate temperatures onthe order of 300° C. while modified spray dryers can achievetemperatures up to about 600° C. However, scale-up of modified systemscan be cost prohibitive. Equipment downstream of the spray dryer canalso impose temperature limitations on the outlet temperature of thesystem that will also limit the maximum conversion temperature.Typically, the maximum particle temperature is assumed to be the outlettemperature for co-current flow spray dryer configurations. Typicalresidence times within a spray dryer are on the order of tens ofseconds.

The lower operating temperature of a spray dryer limits the ability toachieve the chemical reactions during spray conversion. In the presentspray process, the temperature can be significantly higher, but ifcarbon is included as one of the phases, there is an upper temperaturelimit where the carbon will oxidize if an oxidizing environment such asair is used. The upper temperature limit will be higher for the presentprocess because the residence time is shorter.

Atomizers useful with spray dryers include single-fluid and two-fluidnozzles and rotary wheel atomizers. The preferred method for spraydrying the particulate carbon-containing precursors of the presentinvention is a two-fluid nozzle. Rotary wheel atomizers accumulate driedmaterial at the edge requiring frequent cleaning and intermittentoperation. Single-fluid nozzles can be used but require higher liquidflow rates to achieve the same particle size distribution.

A spray dryer system that is useful according to the present inventionis schematically illustrated in FIG. 2. The spray dryer 200 includes aprecursor feed line 202 for delivering liquid precursor to the dryingchamber 204 and an atomizing gas line 203 for atomizing the liquid feed.The liquid precursor is dispersed into droplets through a spray nozzle206, such as the two-fluid nozzle illustrated in FIG. 1. Drying air isintroduced at the top of the chamber 204 through a hot gas inlet 208.The liquid droplets are dried and collected in a cyclone 212.

In the foregoing description of the basic components of a spray dryer,it should be noted that during spray drying the precursor mixtureactually undergoes a chemical conversion. For example, a manganeseprecursor, such as potassium permanganate, is converted to manganeseoxide. The final phase and oxidation state of manganese oxide arecritical to the electrocatalytic activity of the resulting powder. Minorvariations in reaction temperature and precursor composition can resultin powders with different electrocatalytic activities.

It has been advantageously found according to the present invention thatrelatively low conversion temperatures can be used to obtain qualityelectrocatalyst powder. It is preferred that the reaction temperature isnot greater than about 400° C., more preferably not greater than about300° C. and even more preferably not greater than about 200° C. Further,it is preferred that the reaction temperature is at least about 100° C.,preferably at least about 150° C. Increasing the reaction temperature toover 400° C. can remove excess surfactant which may remain on the powderand poison the oxide active sites. However, this is typically notnecessary if the amount of surfactant in the precursor solution is low.

The process of the present invention is particularly well suited for theproduction of finely divided particles having a small weight averageparticle size. In addition to making particles within a desired range ofweight average particle size, the particles may advantageously beproduced with a narrow size distribution, thereby providing sizeuniformity that is desired for many applications. In addition, themethod of the present invention provides significant flexibility forproducing electrocatalyst particles of varying composition,crystallinity, morphology and microstructure.

Referring now to FIG. 3, one embodiment of the process of the presentinvention utilizing ultrasonic fountains is described. A liquid feed102, including the precursor for the desired particles, and a carriergas 104 are fed to an aerosol generator 106 where an aerosol 108 isproduced. The aerosol 108 is then fed to a furnace 110 where liquid inthe aerosol 108 is removed to produce particles 112 that are dispersedin and suspended by gas exiting the furnace 110. The particles 112 arethen collected in a particle collector 114 to produce an electrocatalystpowder batch 116.

As used herein, the liquid feed 102 is a feed that includes a liquid asthe major constituent, such that the feed is a flowable medium. Theliquid feed 102 need not comprise only liquid constituents. The liquidfeed 102 also include particulate material suspended in a liquid phase.The liquid feed 102 must, however, be capable of being atomized to formdroplets of sufficiently small size for preparation of the aerosol 108.Therefore, if the liquid feed 102 includes suspended particles, such assuspended carbon particles, those particles should be relatively smallin relation to the size of droplets in the aerosol 108, as is discussedin more detail hereinbelow.

Frequently, the precursor to one or more of the desired components willbe a material, such as a salt, dissolved in a liquid solvent of theliquid feed 102. The precursor may undergo one or more chemicalreactions in the furnace 110 to form the particles 112. Alternatively,the precursor material may contribute to formation of the particles 112without undergoing a substantial chemical reaction. This could be thecase, for example, when the liquid feed 102 includes, as a precursormaterial, suspended carbon particles that are not substantiallychemically modified in the furnace 110. In any event, the particles 112include at least one component originally contributed by the precursor.Preferred precursors in accordance with the present invention arediscussed above.

The carrier gas 104 may comprise any gaseous medium in which dropletsproduced from the liquid feed 102 may be dispersed in aerosol form. Thecarrier gas 104 may be inert, in that the carrier gas 104 does notparticipate in formation of the particles 112. Alternatively, thecarrier gas may have one or more active components that contribute toformation of the particles 112 in the furnace 110. Preferred carriergases according to the present invention are discussed in more detailhereinbelow.

The aerosol generator 106 atomizes the liquid feed 102 to form dropletsin a manner to permit the carrier gas 104 to sweep the droplets away toform the aerosol 108. An important aspect of the present invention isgeneration of the aerosol 108 with droplets of a small average size and,preferably, a narrow size distribution. In this manner, the particles112 may be produced at a desired small size with a narrow sizedistribution, which is advantageous for many applications.

The aerosol generator 106 is preferably capable of producing the aerosol108 such that it includes droplets having a weight average size in arange having a lower limit of about 1 μm and preferably about 2 μm; andan upper limit of about 20 μm; preferably about 10 μm, more preferablyabout 7 μm and most preferably about 5 μm. The aerosol generator is alsocapable of producing the aerosol 108 such that it includes dropletshaving a narrow size distribution. Preferably, the droplets in theaerosol are such that at least about 70 percent of the droplets aresmaller than about 10 μm and more preferably at least about 70 weightpercent are smaller than about 5 μm. Furthermore, preferably no greaterthan about 30 weight percent, more preferably no greater than about 25weight percent and most preferably no greater than about 20 weightpercent, of the droplets in the aerosol 108 are larger than about twicethe weight average droplet size.

Another important aspect of the present invention is that the aerosol108 may be generated without consuming excessive amounts of the carriergas 104. The aerosol generator 106 is capable of producing the aerosol108 such that it has a high loading, or high concentration, of theliquid feed 102 in droplet form. The capability of the aerosol generator106 to produce a heavily loaded aerosol 108 is surprising given the highdroplet output rate of which the aerosol generator 106 is capable, as isdiscussed more fully below.

The furnace 110, or hot wall reactor, may be any device suitable forheating the aerosol 108 to evaporate liquid from the droplets of theaerosol 108 and thereby form the particles 112. The maximum averagestream temperature, or conversion temperature, refers to the maximumaverage temperature that an aerosol stream attains while flowing throughthe furnace. This is typically determined by a temperature probeinserted into the furnace. Preferred conversion temperatures accordingto the present invention are discussed more fully below.

Although longer residence times are possible, for many applications,residence time in the heating zone of the furnace 110 of shorter thanabout 4 seconds is typical, with shorter than about 2 seconds beingpreferred, shorter than about 1 second being more preferred, shorterthan about 0.5 second being even more preferred, and shorter than about0.2 second being most preferred. The residence time should be longenough, however, to assure that the particles 112 attain the desiredmaximum stream temperature for a given heat transfer rate. In thatregard, with extremely short residence times, higher furnacetemperatures could be used to increase the rate of heat transfer so longas the particles 112 attain a maximum temperature within the desiredstream temperature range.

Typically, the furnace 110 will be a tube-shaped furnace, so that theaerosol 108 moving into and through the furnace does not encounter sharpedges on which droplets could collect. Loss, of droplets to collectionat sharp surfaces results in a lower yield of particles 112. Further,the accumulation of liquid at sharp edges can result in re-release ofundesirably large droplets back into the aerosol 108, which can causecontamination of the particulate product 116 with undesirably largeparticles. Also, over time, such liquid collection at sharp surfaces cancause fouling of process equipment, impairing process performance.

The furnace 110 may include a heating tube made of any suitablematerial. The tube material may be a ceramic material, for example,mullite, silica or alumina. Quartz tubes can also be advantageous.Preferably, the tube is metallic. Advantages of using a metallic tubeare low cost, ability to withstand steep temperature gradients and largethermal shocks, machinability and weldability, and ease of providing aseal between the tube and other process equipment. Disadvantages ofusing a metallic tube include limited operating temperature andincreased reactivity in some reaction systems.

When a metallic tube is used in the furnace 110, it is preferably a highnickel content stainless steel alloy, such as a 330 stainless steel, ora nickel-based super alloy. As noted, one of the major advantages ofusing a metallic tube is that the tube is relatively easy to seal withother process equipment. Flange fittings may be welded directly to thetube for connecting with other process equipment. Metallic tubes aregenerally preferred for making particles that do not require a maximumtube wall temperature of higher than about 1100° C. during particlemanufacture, such as the electrocatalyst particles of the presentinvention.

Also, although the present invention is described with primary referenceto a furnace reactor, which is preferred, it should be recognized that,except as noted, any other thermal reactor, including a flame reactor ora plasma reactor, could be used. A furnace reactor is preferred becauseof the generally even heating characteristic of a furnace for attaininga uniform stream temperature.

The particle collector 114, may be any suitable apparatus for collectingparticles 112 to produce the particulate product 116. One preferredembodiment of the particle collector 114 uses one or more filter toseparate the particles 112 from the gas. Such a filter may be of anytype, including a bag filter. Another preferred embodiment of theparticle collector uses one or more cyclone to separate the particles112. Other apparatus that may be used in the particle collector 114includes an electrostatic precipitator. Particle collection shouldnormally occur at a temperature above the condensation temperature ofthe gas stream in which the particles 112 are suspended. Further,collection should normally be at a temperature that is low enough toprevent significant agglomeration of the particles 112.

Of significant importance to the operation of the process of the presentinvention is the aerosol generator 106, which must be capable ofproducing a high quality aerosol with high droplet loading, aspreviously noted. With reference to FIG. 4, one embodiment of an aerosolgenerator 106 of the present invention is described. The aerosolgenerator 106 includes a plurality of ultrasonic transducer discs 120that are each mounted in a transducer housing 122. The transducerhousings 122 are mounted to a transducer mounting plate 124, creating anarray of the ultrasonic transducer discs 120. Any convenient spacing maybe used for the ultrasonic transducer discs 120. Center-to-centerspacing of the ultrasonic transducer discs 120 of about 4 centimeters isoften adequate. The aerosol generator 106, as shown in FIG. 4, includesforty-nine transducers in a 7×7 array. The array configuration is asshown in FIG. 5, which depicts the locations of the transducer housings122 mounted to the transducer mounting plate 124.

With continued reference to FIG. 4, a separator 126, in spaced relationto the transducer discs 120, is retained between a bottom retainingplate 128 and a top retaining plate 130. Gas delivery tubes 132 areconnected to gas distribution manifolds 134, which have gas deliveryports 136. The gas distribution manifolds 134 are housed within agenerator body 138 that is covered by generator lid 140. A transducerdriver 144, having circuitry for driving the transducer discs 120, iselectronically connected with the transducer discs 120 via electricalcables 146.

During operation of the aerosol generator 106, as shown in FIG. 4, thetransducer discs 120 are activated by the transducer driver 144 via theelectrical cables 146. The transducers preferably vibrate at a frequencyof from about 1 MHz to about 5 MHz, more preferably from about 1.5 MHzto about 3 MHz. Frequently used frequencies are at about 1.6 MHz andabout 2.4 MHz. Furthermore, all of the transducer discs 110 should beoperating at substantially the same frequency when an aerosol with anarrow droplet size distribution is desired. This is important becausecommercially available transducers can vary significantly in thickness,sometimes by as much as 10%, resulting in deviations in the operatingfrequencies. It is preferred, however, that the transducer discs 120operate at frequencies within a range of 5% above and below the mediantransducer frequency, more preferably within a range of 2.5%, and mostpreferably within a range of 1%. This can be accomplished by carefulselection of the transducer discs 120 so that they all preferably havethicknesses within 5% of the median transducer thickness, morepreferably within 2.5%, and most preferably within 1%.

Liquid feed 102 enters through a feed inlet 148 and flows through flowchannels 150 to exit through feed outlet 152. An ultrasonicallytransmissive fluid, typically water, enters through a water inlet 154 tofill a water bath volume 156 and flow through flow channels 158 to exitthrough a water outlet 160. A proper flow rate of the ultrasonicallytransmissive fluid is necessary to cool the transducer discs 120 and toprevent overheating of the ultrasonically transmissive fluid. Ultrasonicsignals from the transducer discs 120 are transmitted, via theultrasonically transmissive fluid, across the water bath volume 156, andultimately across the separator 126, to the liquid feed 102 in flowchannels 150.

The ultrasonic signals from the ultrasonic transducer discs 120 causeatomization cones 162 to develop in the liquid feed 102 at locationscorresponding with the transducer discs 120. Carrier gas 104 isintroduced into the gas delivery tubes 132 and delivered to the vicinityof the atomization cones 162 via gas delivery ports 136. Jets of carriergas exit the gas delivery ports 136 in a direction so as to impinge onthe atomization cones 162, thereby sweeping away atomized droplets ofthe liquid feed 102 that are being generated from the atomization cones162 and creating the aerosol 108, which exits the aerosol generator 106through an aerosol exit opening 164.

Efficient use of the carrier gas 104 is an important aspect of theaerosol generator 106. The embodiment of the aerosol generator 106 shownin FIG. 4 includes two gas exit ports per atomization cone 162, with thegas ports being positioned above the liquid medium 102 over troughs thatdevelop between the atomization cones 162, such that the exiting carriergas 104 is horizontally directed at the surface of the atomization cones162, thereby efficiently distributing the carrier gas 104 to criticalportions of the liquid feed 102 for effective and efficient sweepingaway of droplets as they form about the ultrasonically energizedatomization cones 162. Furthermore, it is preferred that at least aportion of the opening of each of the gas delivery ports 136, throughwhich the carrier gas exits the gas delivery tubes, should be locatedbelow the top of the atomization cones 162 at which the carrier gas 104is directed. This relative placement of the gas delivery ports 136 isvery important to efficient use of carrier gas 104. Orientation of thegas delivery ports 136 is also important. Preferably, the gas deliveryports 136 are positioned to horizontally direct jets of the carrier gas104 at the atomization cones 162. The aerosol generator 106 permitsgeneration of the aerosol 108 with heavy loading with droplets of thecarrier liquid 102, unlike aerosol generator designs that do notefficiently focus gas delivery to the locations of droplet formation.

Another important feature of the aerosol generator 106, as shown in FIG.4, is the use of the separator 126, which protects the transducer discs120 from direct contact with the liquid feed 102, which is often highlycorrosive. The height of the separator 126 above the top of thetransducer discs 120 should normally be kept as small as possible, andis often in the range of from about 1 centimeter to about 2 centimeters.The top of the liquid feed 102 in the flow channels above the tops ofthe ultrasonic transducer discs 120 is typically in a range of fromabout 2 centimeters to about 5 centimeters, whether or not the aerosolgenerator includes the separator 126, with a distance of about 3 to 4centimeters being preferred. Although the aerosol generator 106 could bemade without the separator 126, in which case the liquid feed 102 wouldbe in direct contact with the transducer discs 120, the highly corrosivenature of the liquid feed 102 can often cause premature failure of thetransducer discs 120. The use of the separator 126, in combination withuse of the ultrasonically transmissive fluid in the water bath volume156 to provide ultrasonic coupling, significantly extends the life ofthe ultrasonic transducers 120. One disadvantage of using the separator126, however, is that the rate of droplet production from theatomization cones 162 is reduced, often by a factor of two or more,relative to designs in which the liquid feed 102 is in direct contactwith the ultrasonic transducer discs 102. Even with the separator 126,however, the aerosol generator 106 used with the present invention iscapable of producing a high quality aerosol with heavy droplet loading,as previously discussed. Suitable materials for the separator 126include, for example, polyamides (such as Kapton™ membranes from DuPont)and other polymer materials, glass, and plexiglass. The mainrequirements for the separator 126 are that it be ultrasonicallytransmissive, corrosion resistant and impermeable.

One alternative to using the separator 126 is to bind acorrosion-resistant protective coating onto the surface of theultrasonic transducer discs 120, thereby preventing the liquid feed 102from contacting the surface of the ultrasonic transducer discs 120. Whenthe ultrasonic transducer discs 120 have a protective coating, theaerosol generator 106 will typically be constructed without the waterbath volume 156 and the liquid feed 102 will flow directly over theultrasonic transducer discs 120. Examples of such protective coatingmaterials include platinum, gold, TEFLON™, epoxies and various plastics.Such coating typically significantly extends transducer life. Also, whenoperating without the separator 126, the aerosol generator 106 willtypically produce the aerosol 108 with a much higher droplet loadingthan when the separator 126 is used.

One surprising finding with operation of the aerosol generator 106 ofthe present invention is that the droplet loading in the aerosol may beaffected by the temperature of the liquid feed 102. It has been foundthat when the liquid feed 102 includes an aqueous liquid at an elevatedtemperature, the droplet loading increases significantly. Thetemperature of the liquid feed 102 is preferably higher than about 30°C., more preferably higher than about 35° C. and most preferably higherthan about 40° C. If the temperature becomes too high, however, it canhave a detrimental effect on droplet loading in the aerosol 108.Therefore, the temperature of the liquid feed 102 from which the aerosol108 is made should generally be lower than about 50° C., and preferablylower than about 45° C. The liquid feed 102 may be maintained at thedesired temperature in any suitable fashion. For example, the portion ofthe aerosol generator 106 where the liquid feed 102 is converted to theaerosol 108 could be maintained at a constant elevated temperature.Alternatively, the liquid feed 102 could be delivered to the aerosolgenerator 106 from a constant temperature bath maintained separate fromthe aerosol generator 106. When the ultrasonic generator 106 includesthe separator 126, the ultrasonically transmissive fluid adjacent theultrasonic transducer discs 120 are preferably also at an elevatedtemperature in the ranges discussed for the liquid feed 102.

The design for the aerosol generator 106 based on an array of ultrasonictransducers is versatile and is easily modified to accommodate differentgenerator sizes for different applications. The aerosol generator 106may be designed to include a plurality of ultrasonic transducers in anyconvenient number. Even for smaller scale production, however, theaerosol generator 106 preferably has at least nine ultrasonictransducers, more preferably at least 16 ultrasonic transducers, andeven more preferably at least 25 ultrasonic transducers. For largerscale production, however, the aerosol generator 106 includes at least40 ultrasonic transducers, more preferably at least 100 ultrasonictransducers, and even more preferably at least 400 ultrasonictransducers. In some large volume applications, the aerosol generatormay have at least 1000 ultrasonic transducers.

FIGS. 6–23 show component designs for an aerosol generator 106 includingan array of 400 ultrasonic transducers. Referring first to FIGS. 6 and7, the transducer mounting plate 124 is shown with a design toaccommodate an array of 400 ultrasonic transducers, arranged in foursubarrays 170 of 100 ultrasonic transducers-each. The transducermounting plate 124 has integral vertical walls 172 for containing theultrasonically transmissive fluid, typically water, in a water bathsimilar to the water bath volume 156 described previously with referenceto FIG. 4.

As shown in FIGS. 6 and 7, four hundred transducer mounting receptacles174 are provided in the transducer mounting plate 124 for mountingultrasonic transducers for the desired array. With reference to FIG. 8the profile of an individual transducer mounting receptacle 174 isshown. A mounting seat 176 accepts an ultrasonic transducer formounting, with a mounted ultrasonic transducer being held in place viascrew holes 178. Opposite the mounting receptacle 176 is a flaredopening 180 through which an ultrasonic signal may be transmitted forthe purpose of generating the aerosol 108, as previously described withreference to FIG. 4.

A preferred transducer mounting configuration, however, is shown in FIG.9 for another configuration for the transducer mounting plate 124. Asillustrated in FIG. 9, an ultrasonic transducer disc 120 is mounted tothe transducer mounting plate 124 by use of a compression screw 177threaded into a threaded receptacle 179. The compression screw 177 bearsagainst the ultrasonic transducer disc 120, causing an o-ring 181,situated in an o-ring seat 182 on the transducer mounting plate, to becompressed to form a seal between the transducer mounting plate 124 andthe ultrasonic transducer disc 120. This type of transducer mounting isparticularly preferred when the ultrasonic transducer disc 120 includesa protective surface coating, as discussed previously, because the sealof the o-ring to the ultrasonic transducer disc 120 will be inside ofthe outer edge of the protective seal, thereby preventing liquid frompenetrating under the protective surface coating from the edges of theultrasonic transducer disc 120.

Referring now to FIG. 10, the bottom retaining plate 128 for a 400transducer array is shown having a design for mating with the transducermounting plate 124 (shown in FIGS. 6–7). The bottom retaining plate 128has eighty openings 184, arranged in four subgroups 186 of twentyopenings 184 each. Each of the openings 184 corresponds with five of thetransducer mounting receptacles 174 (shown in FIGS. 6 and 7) when thebottom retaining plate 128 is mated with the transducer mounting plate124 to create a volume for a water bath between the transducer mountingplate 124 and the bottom retaining plate 128. The openings 184,therefore, provide a pathway for ultrasonic signals generated byultrasonic transducers to be transmitted through the bottom retainingplate.

Referring now to FIGS. 11 and 12, a liquid feed box 190 for a 400transducer array is shown having the top retaining plate 130 designed tofit over the bottom retaining plate 128 (shown in FIG. 10), with aseparator 126 (not shown) being retained between the bottom retainingplate 128 and the top retaining plate 130 when the aerosol generator 106is assembled. The liquid feed box 190 also includes vertically extendingwalls 192 for containing the liquid feed 102 when the aerosol generatoris in operation. Also shown in FIGS. 11 and 12 is the feed inlet 148 andthe feed outlet 152. An adjustable weir 198 determines the level ofliquid feed 102 in the liquid feed box 190 during operation of theaerosol generator 106.

The top retaining plate 130 of the liquid feed box 190 has eightyopenings 194 therethrough, which are arranged in four subgroups 196 oftwenty openings 194 each. The openings 194 of the top retaining plate130 correspond in size with the openings 184 of the bottom retainingplate 128 (shown in FIG. 10). When the aerosol generator 106 isassembled, the openings 194 through the top retaining plate 130 and theopenings 184 through the bottom retaining plate 128 are aligned, withthe separator 126 positioned therebetween, to permit transmission ofultrasonic signals when the aerosol generator 106 is in operation.

Referring now to FIGS. 11–13, a plurality of gas tube feed-through holes202 extend through the vertically extending walls 192 to either side ofthe assembly including the feed inlet 148 and feed outlet 152 of theliquid feed box 190. The gas tube feed-through holes 202 are designed topermit insertion therethrough of gas tubes 208 of a design as shown inFIG. 13. When the aerosol generator 106 is assembled, a gas tube 208 isinserted through each of the gas tube feed-through holes 202 so that gasdelivery ports 136 in the gas tube 208 will be properly positioned andaligned adjacent the openings 194 in the top retaining plate 130 fordelivery of gas to atomization cones that develop in the liquid feed box190 during operation of the aerosol generator 106. The gas deliveryports 136 are typically holes having a diameter of from about 1.5millimeters to about 3.5 millimeters.

Referring now to FIG. 14, a partial view of the liquid feed box 190 isshown with gas tubes 208A, 208B and 208C positioned adjacent to theopenings 194 through the top retaining plate 130. Also shown in FIG. 14are the relative locations that ultrasonic transducer discs 120 wouldoccupy when the aerosol generator-106 is assembled. As seen in FIG. 14,the gas tube 208A, which is at the edge of the array, has five gasdelivery ports 136. Each of the gas delivery ports 136 is positioned todivert carrier gas 104 to a different one of atomization cones thatdevelop over the array of ultrasonic transducer discs 120 when theaerosol generator 106 is operating. The gas tube 208B, which is one rowin from the edge of the array, is a shorter tube that has ten gasdelivery ports 136, five each on opposing sides of the gas tube 208B.The gas tube 208B, therefore, has gas delivery ports 136 for deliveringgas to atomization cones corresponding with each of ten ultrasonictransducer discs 120. The third gas tube, 208C, is a longer tube thatalso has ten gas delivery ports 136 for delivering gas to atomizationcones corresponding with ten ultrasonic transducer discs 120. The designshown in FIG. 14, therefore, includes one gas delivery port perultrasonic transducer disc 120. Although this is a lower density of gasdelivery ports 136 than for the embodiment of the aerosol generator 106shown in FIG. 4, which includes two gas delivery ports per ultrasonictransducer disc 120, the design shown in FIG. 14 is, nevertheless,capable of producing a dense, high-quality aerosol without unnecessarywaste of gas.

Referring now to FIG. 15, the flow of carrier gas 104 relative toatomization cones 162 during operation of the aerosol generator 106having a gas distribution configuration to deliver carrier gas 104 fromgas delivery ports on both sides of the gas tubes 208, as was shown forthe gas tubes 208A, 208B and 208C in the gas distribution configurationshown in FIG. 13. The carrier gas 104 sweeps both directions from eachof the gas tubes 208.

An alternative, and preferred, flow for carrier gas 104 is shown in FIG.16. As shown in FIG. 16, carrier gas 104 is delivered from only one sideof each of the gas tubes 2 b 8. This results in a sweep of carrier gasfrom all of the gas tubes 208 toward a central area 212. This results ina more uniform flow pattern for aerosol generation that maysignificantly enhance the efficiency with which the carrier gas 104 isused to produce an aerosol. The aerosol that is generated, therefore,tends to be more heavily loaded with liquid droplets.

Another configuration for distributing carrier gas in the aerosolgenerator 106 is shown in FIGS. 17 and 18. In this configuration, thegas tubes 208 are hung from a gas distribution plate 216 adjacent gasflow holes 218 through the gas distribution plate 216. In the aerosolgenerator 106, the gas distribution plate 216 would-be mounted above theliquid feed, with the gas flow holes positioned to each correspond withan underlying ultrasonic transducer. Referring specifically to FIG. 18,when the ultrasonic generator 106 is in operation, atomization cones 162develop through the gas flow holes 218, and the gas tubes 208 arelocated such that carrier gas 104 exiting from ports in the gas tubes208 impinge on the atomization cones and flow upward through the gasflow holes. The gas flow holes 218, therefore, act to assist inefficiently distributing the carrier gas 104 about the atomization cones162 for aerosol formation. It should be appreciated that the gasdistribution plates 218 can be made to accommodate any number of the gastubes 208 and gas flow holes 218. For convenience of illustration, theembodiment shown in FIGS. 17 and 21 shows a design having only two ofthe gas tubes 208 and only 16 of the gas flow holes 218. Also, it shouldbe appreciated that the gas distribution plate 216 could be used alone,without the gas tubes 208. In that case, a slight positive pressure ofcarrier gas 104 would be maintained under the gas distribution plate 216and the gas flow holes 218 would be sized to maintain the propervelocity of carrier gas 104 through the gas flow holes 218 for efficientaerosol generation. Because of the relative complexity of operating inthat mode, however, it is not preferred.

Aerosol generation may also be enhanced through mounting of ultrasonictransducers at a slight angle and directing the carrier gas at resultingatomization cones such that the atomization cones are tilting in thesame direction as the direction of flow of carrier gas. Referring toFIG. 19, an ultrasonic transducer disc 120 is shown. The ultrasonictransducer disc 120 is tilted at a tilt angle 114 (typically less than10 degrees), so that the atomization cone 162 will also have a tilt. Itis preferred that the direction of flow of the carrier gas 104 directedat the atomization cone 162 is in the same direction as the tilt of theatomization cone 162.

Referring now to FIGS. 20 and 21, a gas manifold 220 is shown fordistributing gas to the gas tubes 208 in a 400 transducer array design.The gas manifold 220 includes a gas distribution box 222 and pipingstubs 224 for connection with gas tubes 208 (shown in FIG. 13). Insidethe gas distribution box 222 are two gas distribution plates 226 thatform a flow path to assist in distributing the gas equally throughoutthe gas distribution box 222, to promote substantially equal delivery ofgas through the piping stubs 224. The gas manifold 220, as shown inFIGS. 20 and 21, is designed to feed eleven gas tubes 208. For the 400transducer design, a total of four gas manifolds 220 are required.

Referring now to FIGS. 22 and 23, the generator lid 140 is shown for a400 transducer array design. The generator lid 140 mates with and coversthe liquid feed box 190 (shown in FIGS. 11 and 12). The generator lid140, as shown in FIGS. 22 and 23, has a hood design to permit easycollection of the aerosol 108 without subjecting droplets in the aerosol108 to sharp edges on which droplets may coalesce and be lost, andpossibly interfere with the proper operation of the aerosol generator106. When the aerosol generator 106 is in operation, the aerosol 108would be withdrawn via the aerosol exit opening 164 through thegenerator cover 140.

It is important that the aerosol stream fed to the furnace 110 have ahigh droplet flow rate and high droplet loading as would be required formost industrial applications. With the present invention, the aerosolstream fed to the furnace preferably includes a droplet flow of greaterthan about 0.5 liters per hour, more preferably greater than about 2liters per hour, still more preferably greater than about 5 liters perhour, even more preferably greater than about 10 liters per hour,particularly greater than about 50 liters per hour and most preferablygreater than about 100 liters per hour; and with the droplet loadingbeing typically greater than about 0.04 milliliters of droplets perliter of carrier gas, preferably greater than about 0.083 milliliters ofdroplets per liter of carrier gas 104, more preferably greater thanabout 0.167 milliliters of droplets per liter of carrier gas 104, stillmore preferably greater than about 0.25 milliliters of droplets perliter of carrier gas 104, particularly greater than about 0.33milliliters of droplets per liter of carrier gas 104 and most preferablygreater than about 0.83 milliliters of droplets per liter of carrier gas104.

As discussed previously, the aerosol generator 106 of the presentinvention produces a concentrated, high quality aerosol of micro-sizeddroplets having a relatively narrow size distribution. It has beenfound, however, that for many applications the process of the presentinvention can be enhanced by further classifying by size the droplets inthe aerosol 108 prior to introduction of the droplets into the furnace110. In this manner, the size and size distribution of particles in theparticulate product 116 are further controlled.

Referring now to FIG. 24, a process flow diagram is shown for oneembodiment of the process of the present invention including suchdroplet classification. As shown in FIG. 24, the aerosol 108 from theaerosol generator 106 goes to a droplet classifier 280 where oversizeddroplets are removed from the aerosol 108 to prepare a classifiedaerosol 282. Liquid 284 from the oversized droplets that are beingremoved is drained from the droplet classifier 280. This drained liquid284 may advantageously be recycled for use in preparing additionalliquid feed 102.

Any suitable droplet classifier may be used for removing droplets abovea predetermined size. For example, a cyclone could be used to removeover-size droplets. A preferred droplet classifier for manyapplications, however, is an impactor. One embodiment of an impactor foruse with the present invention will now be described with reference toFIGS. 25–29.

As seen in FIG. 25, an impactor 288 has disposed in a flow conduit 286 aflow control plate 290 and an impactor plate assembly 292. The flowcontrol plate 290 is conveniently mounted on a mounting plate 294.

The flow control plate 290 is used to channel the flow of the aerosolstream toward the impactor plate assembly 292 in a manner withcontrolled flow characteristics that are desirable for proper impactionof oversize droplets on the impactor plate assembly 292 for removalthrough the drains 296 and 314. One embodiment of the flow control plate290 is shown in FIG. 26. The flow control plate 290 has an array ofcircular flow ports 296 for channeling flow of the aerosol 108 towardsthe impactor plate assembly 292 with the desired flow characteristics.

Details of the mounting plate 294 are shown in FIG. 27. The mountingplate 294 has a mounting flange 298 with a large diameter flow opening300 passing therethrough to permit access of the aerosol 108 to the flowports 296 of the flow control plate 290 (shown in FIG. 26).

Referring now to FIGS. 28 and 29, one embodiment of an impactor plateassembly 292 is shown. The impactor plate assembly 292 includes animpactor plate 302 and mounting brackets 304 and 306 used to mount theimpactor plate 302 inside of the flow conduit 286. The impactor plate302 and the flow channel plate 290 are designed so that droplets largerthan a predetermined size will have momentum that is too large for thoseparticles to change flow direction to navigate around the impactor plate302.

During operation of the impactor 288, the aerosol 108 from the aerosolgenerator 106 passes through the upstream flow control plate 290. Mostof the droplets in the aerosol navigate around the impactor plate 302and exit the impactor 288 through the downstream flow control plate 290in the classified aerosol 282. Droplets in the aerosol 108 that are toolarge to navigate around the impactor plate 302 will impact on theimpactor plate 302 and drain through the drain 296 to be collected withthe drained liquid 284 (as shown in FIG. 25).

The configuration of the impactor plate 302 shown in FIG. 24 representsonly one of many possible configurations for the impactor plate 302. Forexample, the impactor 288 could include an upstream flow control plate290 having vertically extending flow slits therethrough that are offsetfrom vertically extending flow slits through the impactor plate 302,such that droplets too large to navigate the change in flow due to theoffset of the flow slits between the flow control plate 290 and theimpactor plate 302 would impact on the impactor plate 302 to be drainedaway.

The classification size, also called the classification cut point, isthat size at which half of the droplets of that size are removed andhalf of the droplets of that size are retained. Depending upon thespecific application the droplet classification size may be varied, suchas by changing the spacing between the impactor plate 302 and the flowcontrol plate 290 or increasing or decreasing aerosol velocity throughthe jets in the flow control plate 290. Minimizing the removal of liquidfeed 102 from the aerosol 108 is particularly important for commercialapplications to increase the yield of high quality particulate product116. It should be noted that because of the superior performance of theaerosol generator 106, it is typically not required to use an impactoror other droplet classifier to obtain a suitable aerosol. This is amajor advantage, because the added complexity and liquid lossesaccompanying use of an impactor may often be avoided with the process ofthe present invention.

With some applications of the process of the present invention, it maybe possible to collect the particles 112 directly from the output of thefurnace 110. More often, however, it will be desirable to cool theparticles 112 exiting the furnace 110 prior to collection of theparticles 112 in the particle collector 114. Referring now to FIG. 30,one embodiment of the process of the present invention is shown in whichthe particles 112 exiting the furnace 110 are sent to a particle cooler320 to produce a cooled particle stream 322, which is then fed to theparticle collector 114. According to the present invention, a gas quenchapparatus is provided for use as the particle cooler 320 thatsignificantly reduces thermophoretic losses compared to a traditionalheat exchanger.

Referring now to FIGS. 31–33, one embodiment of a gas quench cooler 330is shown. The gas quench cooler includes a perforated conduit 332 housedinside of a cooler housing 334 with an annular space 336 located betweenthe cooler housing 334 and the perforated conduit 332. In fluidcommunication with the annular space 336 is a quench gas inlet box 338,inside of which is disposed a portion of an aerosol outlet conduit 340.The perforated conduit 332 extends between the aerosol outlet conduit340 and an aerosol inlet conduit 342. Attached to an opening into thequench gas inlet box 338 are two quench gas feed tubes 344. Referringspecifically to FIG. 33, the perforated tube 332 is shown. Theperforated tube 332 has a plurality of openings 345. The openings 345,when the perforated conduit 332 is assembled into the gas quench cooler330, permit the flow of quench gas 346 from the annular space 336 intothe interior space 348 of the perforated conduit 332. Although theopenings 345 are shown as being round holes, any shape of opening couldbe used, such as slits. Also, the perforated conduit 332 could be aporous screen. Two heat radiation shields 347 prevent downstream radiantheating from the furnace. In most instances, however, it will not benecessary to include the heat radiation shields 347, because downstreamradiant heating from the furnace is normally not a significant problem.Use of the heat radiation shields 347 is not preferred due toparticulate losses that accompany their use.

With continued reference to FIGS. 31–33, operation of the gas quenchcooler 330 will now be described. During operation, the particles 112,carried by and dispersed in a gas stream, enter the gas quench cooler330 through the aerosol inlet conduit 342 and flow into the interiorspace 348 of perforated conduit 332. Quench gas 346 is introducedthrough the quench gas feed tubes 344 into the quench gas inlet box 338.Quench gas 346 entering the quench gas inlet box 338 encounters theouter surface of the aerosol outlet conduit 340, forcing the quench gas346 to flow, in a spiraling, swirling manner, into the annular space336, where the quench gas 346 flows through the openings 345 through thewalls of the perforated conduit 332. Preferably, the gas 346 retainssome swirling motion even after passing into the interior space 348. Inthis way, the particles 112 are quickly cooled with low losses ofparticles to the walls of the gas quench cooler 330. In this manner, thequench gas 346 enters in a radial direction into the interior space 348of the perforated conduit 332 around the entire periphery, orcircumference, of the perforated conduit 332 and over the entire lengthof the perforated conduit 332. The cool quench gas 346 mixes with andcools the hot particles 112, which then exit through the aerosol outletconduit 340 as the cooled particle stream 322. The cooled particlestream 322 can then be sent to the particle collector 114 for particlecollection. The temperature of the cooled particle stream 322 iscontrolled by introducing more or less quench gas.

Also, as shown in FIG. 32, the quench gas 346 is fed into the quenchcooler 330 in counter flow to flow of the particles. Alternatively, thequench cooler could be designed so that the quench gas 346 is fed intothe quench cooler in concurrent flow with the flow of the particles 112.The amount of quench gas 346 fed to the gas quench cooler 330 willdepend upon the specific material being made and the specific operatingconditions. The quantity of quench gas 346 used, however, must besufficient to reduce the temperature of the aerosol steam including theparticles 112 to the desired temperature. Typically, the particles 112are cooled to a temperature at least below about 200° C., and oftenlower. The only limitation on how much the particles 112 are cooled isthat the cooled particle stream 322 must be at a temperature that isabove the condensation temperature for water as another condensablevapor in the stream. The temperature of the cooled particle stream 322is often at a temperature of from about 50° C. to about 120° C.

Because of the entry of quench gas 346 into the interior space 348 ofthe perforated conduit 322 in a radial direction about the entirecircumference and length of the perforated conduit 322, a buffer of thecool quench gas 346 is formed about the inner wall of the perforatedconduit 332, thereby significantly inhibiting the loss of hot particles112 due to thermophoretic deposition on the cool wall of the perforatedconduit 332. In operation, the quench gas 346 exiting the openings 345and entering into the interior space 348 should have a radial velocity(velocity inward toward the center of the circular cross-section of theperforated conduit 332) of larger than the thermophoretic velocity ofthe particles 112 inside the perforated conduit 332 in a directionradially outward toward the perforated wall of the perforated conduit332.

As seen in FIGS. 31–33, the gas quench cooler 330 includes a flow pathfor the particles 112 through the gas quench cooler of a substantiallyconstant cross-sectional shape and area. Preferably, the flow paththrough the gas quench cooler 330 will have the same cross-sectionalshape and area as the flow path through the furnace 110 and through theconduit delivering the aerosol 108 from the aerosol generator 106 to thefurnace 110. In one embodiment, however, it may be necessary to reducethe cross-sectional area available for flow prior to the particlecollector 114. This is the case, for example, when the particlecollector includes a cyclone for separating particles in the cooledparticle stream 322 from gas in the cooled particle stream 322. This isbecause of the high inlet velocity requirements into cyclone separators.

Referring now to FIG. 34, one embodiment of the gas quench cooler 330 isshown in combination with a cyclone separator 392. The perforatedconduit 332 has a continuously decreasing cross-sectional area for flowto increase the velocity of flow to the proper value for the feed tocyclone separator 392. Attached to the cyclone separator 392 is a bagfilter 394 for final clean-up of overflow from the cyclone separator392. Separated particles exit with underflow from the cyclone separator392 and may be collected in any convenient container. The use of cycloneseparation is particularly preferred for powder having a weight averagesize of larger than about 1 μm, although a series of cyclones maysometimes be needed to obtain the desired degree of separation. Cycloneseparation is particularly preferred for powders having a weight averagesize of larger than about 1.5 μm.

In a further embodiment of the present invention, following preparationof the particles 112 in the furnace 110, the particles 112 may then bestructurally modified to impart desired physical properties prior toparticle collection. Referring now to FIG. 35, one embodiment of theprocess of the present invention is shown including such structuralparticle modification. The particles 112 exiting the furnace 110 go to aparticle modifier 360 where the particles are structurally modified toform modified particles 362, which are then sent to the particlecollector 114 for preparation of the particulate product 116. Theparticle modifier 360 is typically a furnace, such as an annealingfurnace, which may be integral with the furnace 110 or may be a separateheating device. Regardless, it is important that the particle modifier360 have temperature control that is independent of the furnace 110, sothat the proper conditions for particle modification may be providedseparate from conditions required of the furnace 110 to prepare theparticles 112. The particle modifier 360, therefore, typically providesa temperature controlled environment and necessary residence time toeffect the desired structural modification of the particles 112.

The foregoing methods produce fine powders having a small particle size,spherical morphology and a well-controlled chemistry. FIG. 36 is an SEMphotomicrograph (×10 k) of TEFLON coated carbon particles according toan embodiment of the present invention, which are useful for forming ahydrophobic layer in a thin film battery according to the presentinvention. FIG. 37 is an SEM photomicrograph (×2.0 k) of a bifunctionalelectrocatalyst (Ni—Co—O). In both instances, the particles arespherical with a well controlled size.

The fine powders of the present invention can be deposited onto devicesurfaces or substrates by a number of different deposition methods whichinvolve the direct deposition of the dry powder such as dusting,electrophotographic or electrostatic precipitation. Other depositionmethods involve liquid vehicles such as ink jet printing, syringedispense, toner deposition, slurry deposition, paste-based methods andelectrophoresis. In all these deposition methods, the powders accordingto the present invention have a number of advantages over the powdersproduced by other methods. For example, small, spherical, narrow sizedistribution particles are more easily dispersed in liquid vehicles,they remain dispersed for a longer period of time and allow depositionof smoother and finer features compared to powders made by alternativemethods.

In many applications, the electrocatalyst powders are formed into alayer, often in combination with other materials as a component of adevice such as a fuel cell or battery. The method by which thesematerials are deposited has a strong influence on the characteristics ofthe deposited layer. In turn, the characteristics of the deposited layeralso has a strong influence on the performance of the device. Layercharacteristics that are important include average thickness, porosity,compositional homogeneity, nature of the interface with other layers,control over the compositional gradient within a layer and thehydrophobicity, hydrophilicity, wettability and accessibility of thesurface.

The electrocatalyst powders according to the present exhibit a highcatalytic activity and also have a microstructure which enables them tobe formed into layers by methods that are not useful withelectrocatalyst powders having different characteristics. The highcatalytic activity enables thinner layers of these materials to bedeposited since a reduced mass of the electrocatalyst is required toachieve the same level of performance. However, it is also importantthat in the process of printing the layer, the performance advantages ofthe powders is retained in the layers, for example access to theporosity of the individual particles.

One way of depositing the fine powders of the present invention is toapply the powders to a substrate through the use of a thick-film paste.In the thick film process, a viscous paste that includes a functionalparticulate phase (e.g., a carbon composite powder) is screen printedonto a substrate. More particularly, a porous screen fabricated fromstainless steel, polyester, nylon or similar inert material is stretchedand attached to a rigid frame. A predetermined pattern is formed on thescreen corresponding to the pattern to be printed. For example, a UVsensitive emulsion can be applied to the screen and exposed through apositive or negative image of the design pattern. The screen is thendeveloped to remove portions of the emulsion in the pattern regions.

The screen is then affixed to a screen printing device and the thickfilm paste is deposited on top of the screen. The substrate to beprinted is then positioned beneath the screen and the paste is forcedthrough the screen and onto the substrate by a squeegee that traversesthe screen. Thus, a pattern of traces and/or pads of the paste materialis transferred to the substrate. The substrate with the paste applied ina predetermined pattern is then subjected to a drying and firingtreatment to solidify and adhere the paste to the substrate.

Thick film pastes have a complex chemistry and generally include afunctional phase, a binder phase and an organic vehicle phase. Thefunctional phase include the electrocatalyst powders of the presentinvention. The binder phase can be, for example, a mixture of carbon,metal oxide or glass frit powders. PbO based glasses are commonly usedas binders. The function of the binder phase is to control the sinteringof the film and assist the adhesion of the functional phase to thesubstrate and/or assist in the sintering of the functional phase.Reactive compounds can also be included in the paste to promoteadherence of the functional phase to the substrate.

Thick film pastes also include an organic vehicle phase that is amixture of solvents, polymers, resins and other organics whose mainfunction is to provide the appropriate rheology (flow properties) to thepaste. The liquid solvent assists in mixing of the components into ahomogenous paste and substantially evaporates upon application of thepaste to the substrate. Usually the solvent is a volatile liquid such asmethanol, ethanol, terpineol, butyl carbitol, butyl carbitol acetate,aliphatic alcohols, esters, acetone and the like. The other organicvehicle components can include thickeners (sometimes referred to asorganic binders), stabilizing agents, surfactants, wetting agents andthe like. Thickeners provide sufficient viscosity to the paste and alsoacts as a binding agent in the unfired state. Examples of thickenersinclude ethyl cellulose, polyvinyl acetates, resins such as acrylicresin, cellulose resin, polyester, polyamide and the like. Thestabilizing agents reduce oxidation and degradation, stabilize theviscosity or buffer the pH of the paste. For example, triethanolamine isa common stabilizer. Wetting agents and surfactants are well known inthe thick film paste art and can include triethanolamine and phosphateesters.

The different components of the thick film paste are mixed in thedesired proportions in order to produce a substantially homogenous blendwherein the functional phase is well dispersed throughout the paste.Typically, the thick film paste will include from about 5 to about 95weight percent such as from about 60 to 85 weight percent, of thefunctional phase, including the carbon composite powders of the presentinvention.

Examples of thick film pastes are disclosed in U.S. Pat. Nos. 4,172,733;3,803,708; 4,140,817; and 3,816,097 all of which are incorporated hereinby reference in their entirety.

Some applications of thick film pastes require higher tolerances thancan be achieved using standard thick-film technology, as is describedabove. As a result, some thick film pastes have photo-imaging capabilityto enable the formation of lines and traces with decreased width andpitch (distance between lines). In this type of process, a photoactivethick film paste is applied to a substrate substantially as is describedabove. The paste can include, for example, a liquid vehicle such aspolyvinyl alcohol, that is not cross-linked. The paste is then dried andexposed to ultraviolet light through a patterned photomask to polymerizethe exposed portions of paste. The paste is then developed to removeunwanted portions of the paste. This technology permits higher densitylines and features to be formed. The combination of the foregoingtechnology with the composite powders of the present invention permitsthe fabrication of devices with higher resolution and tolerances ascompared to conventional technologies using conventional powders.

In addition, a laser can be used instead of ultraviolet light through amask. The laser can be scanned over the surface in a pattern therebyreplacing the need for a mask. The laser light is of sufficiently lowintensity that it does not heat the glass or polymer above its softeningpoint. The unirradiated regions of the paste can then be removed leavinga pattern. Likewise, conventional paste technology utilizes heating ofthe substrate to remove the vehicle from a paste and to fuse particlestogether or modify them in some other way. A laser can be used tolocally heat a conventionally applied paste layer wherein the laser isscanned over the paste layer to form a pattern. The laser heating isconfined to the paste layer and drives out the paste vehicle and heatsthe powder in the paste without appreciably heating the substrate. Thisallows heating of particles, delivered using pastes, without damaging aglass or even polymeric substrate.

It is particularly advantageous according to the present invention toprint the layers containing the powders of the present invention using adirect-write tool. There are a number of advantages of constructing anenergy device such as a battery or fuel cell using direct-write methods.Direct-write methods enable the formation of layers that are thinner andwith smaller feature sizes than those that can be produced bymanufacturing methods such as rolling and pressing. The thinner layersresult in reduced mass and volume and therefore an increase in thevolumetric and gravimetric energy density of the battery. The thindevices can be incorporated into unusual vehicles or be directlyintegrated with electronic devices to form compact, self-containedoperational systems.

Thinner layers can also facilitate faster transport of chemical speciessuch as ions, electrons and gases due to the reduced diffusionaldistances. This can lead to improved battery performance where, forexample, the diffusion of a chemical species is otherwise a ratelimiting factor. This is the case in metal-air batteries where thetransport of O₂ or hydroxide ion in the air electrode can be ratelimiting. Shorter diffusional distances and lower diffusional barrierswill lead to a higher rate of drain for this type of device. Thedischarge rate can also be improved.

Direct-write methods can also facilitate better control over theconstruction of interfaces and layer compositions giving rise totailored gradients in composition and layer surface morphology thatfacilitate chemical transport and electrochemical reactions.

Certain direct-write methods facilitate the construction of featureswith combined functionalities such that multiple layers may be combinedinto a single layer with multiple functions to provide benefits in bothperformance and energy density.

To be deposited using a direct-write tool, the particles must be carriedin a liquid vehicle. The particles should remain well-dispersed in theliquid vehicle for extended periods of time and therefore the cartridgeor reservoir into which the suspension is placed will have a longshelf-life. In some instances, substantially fully dense particles canbe adequately dispersed and suspended. Depending upon the density of theparticle compound, however, particles with a high density relative tothe liquid in which they are dispersed and with a size in excess ofabout 0.5 μm cannot be suspended in a liquid that has a sufficiently lowviscosity to be deposited using a direct-write tool, particularly anink-jet device. In most cases, the apparent density of the particlesmust therefore be substantially lower than the theoretical density.

More specifically, it is desirable to maintain a substantially neutralbuoyancy of the particles in the suspension while maintaining arelatively large physical size. The buoyancy is required for inkstability while the larger size maintains ink properties, such asviscosity, within useful ranges. Stated another way, it is desirable toprovide particles having a low settling velocity but with a sufficientlylarge particle size. The settling velocity of the particles isproportional to the apparent density of the particle (ρ_(s)) minus thedensity of the liquid (ρ_(L)). Ideally, the fine particles will have anapparent density that is approximately equal to the density of theliquid, which is typically about 1 g/cm³ (i.e., the density of water).Since a compound such as an oxide has a theoretical density (ρ_(p)) inthe range of from about 3 to about 7 g/cm³, it is preferable that theapparent density of such particles be a small percentage of thetheoretical density. According to one embodiment, the particles have anapparent density that is not greater than about 50 percent of theparticles theoretical density, more preferably not greater than about 20percent of the theoretical density. Such particles would have smallapparent sizes when measured by settling techniques, but larger sizeswhen measured by optical techniques.

In the case of electrocatalyst powders, especially carbon-basedelectrocatalyst powders, they are designed to have a high degree ofporosity and therefore relatively low density. This aids in thesuspendability of these powders in low viscosity inks having a highsolids loading.

Some electrocatalyst formulations may be comprised of material with arelatively high density. One preferred method for obtaining a reducedapparent density of the fine particles according to the presentinvention is to produce particles having either a hollow or a porousmicrostructure (or a combination thereof). Hollow electrocatalystparticles might include carbon, metal or metal oxide based materialswhere the surface area of these materials is high with a desire tomaintain a relatively low apparent density. That is, one preferredparticle morphology is a particle comprised of a dense shell having aninner radius and an outer radius. Preferably, the shell has a highdensity and is substantially impermeable. Assuming that air fills theinterior of such a hollow particle, the equation representing theconditions for neutral buoyancy can be written:

$r_{2} = {\lbrack \sqrt[3]{\frac{\rho_{p}}{\rho_{p} - 1}} \rbrack r_{1}}$

-   -   where:        -   r₂=outer radius        -   r₁=inner radius        -   ρ_(L)=1 (water)        -   ρ_(p)=theoretical density of the particle

For example, if a hollow particle has an outer radius of 2 μm (4 μmdiameter) and a density of 5 g/cm³, then the optimum average wallthickness would be about 0.15 μm for the particle to be neutrallybuoyant in a liquid having a density of 1 g/cm³. According to onepreferred embodiment, the hollow particles have an average wallthickness that is not greater than about 10 percent of the particlediameter, and more preferably not greater than about 5 percent of theparticle diameter.

It will be appreciated that other particle morphologies can be utilizedwhile maintaining an apparent density within the desired range. Forexample, the electrocatalyst particles can have a sufficient amount ofporosity to yield a particle having an apparent density that is lowerthan the theoretical density. Open (surface) porosity can also decreasethe apparent density if the surface tension of the liquid medium doesnot permit penetration of the surface pores by the liquid.

Thus, the fine particles according to the present invention have a lowsettling velocity in the liquid medium. The settling velocity accordingto Stokes Law is defined as:

-   -   where

$V = \frac{{D_{st}^{2}( {\rho_{s} - \rho_{l}} )}g}{18\;\eta}$

-   -   -   D_(st)=Stokes diameter        -   η=fluid viscosity        -   ρ_(s)=apparent density of the particle        -   ρ_(l)=density of the liquid        -   V=settling velocity        -   g=acceleration due to gravity

Preferably, the average settling velocity of the particles issufficiently low such that the suspensions have a useful shelf-lifewithout the necessity of frequent mixing. It is preferred that a largemass fraction of the particles, such as at least about 50 weight percentremains suspended in the liquid. The particles preferably have anaverage settling velocity that is not greater than 50 percent, morepreferably not greater than 20 percent, of a theoretically denseparticle of the same composition. Further, the particles can becompletely redispersed after settling, such as by mixing, to provide thesame particle size distribution in suspension as measured beforesettling.

In one embodiment, the electrocatalyst particles can include ahydrophilic compound, such as silica (SiO₂), hydrophilic carbon, anothermetal oxide or a surface modification agent/surfactant, to enhance thedispersion characteristics of the powder in the liquid, provided thatsuch an additive does not interfere with the electrocatalytic propertiesof the particles. Preferably, the hydrophilic compound is found on atleast a portion of the surface of the particle. As is discussed in moredetail above, the powders according to the present invention arepreferably produced utilizing a spray pyrolysis or spray conversiontechnique, typically in the presence of water vapor. The presence ofwater vapor during formation of the particles can advantageously resultin a hydroxylated particle surface. Such a hydroxylated surface providesfurther hydrophilicity of the particle, aids in the dispersion of theparticles and reduces the number of particle agglomerates in the liquidsuspension.

A direct-write deposition method according to the present invention isillustrated schematically in FIG. 38. In FIG. 38( a), a fine powder 1002is dispersed in an organic vehicle 1004 including water and variousorganics to aid in the dispersion of the fine powder 1002. Thedirect-write tool 1006 ejects the suspension through an orifice and ontoa substrate 1008. After deposition the substrate 1008 is thermallytreated 1010 to remove the liquid vehicle 1004 including the organicsand deposit a thin layer of fine particles 1002.

In the embodiment illustrated in FIG. 38( b), the particles 1012 aredispersed in a liquid vehicle 1014 which include water, organics and atleast one molecular precursor to a compound or a metal. The liquidsuspension including the particles 1012 and the precursor-containingliquid vehicle 1014 are deposited using a direct-write tool 1016 onto asubstrate 1018. After deposition, the substrate 1018 is thermallytreated 1020 to remove liquids and convert the precursors to theirrespective compound or metal. The resulting layer 1022 includesparticles dispersed throughout a film of the compound or metal.

As used herein, a direct-write tool is a device that deposits a liquidor liquid suspension onto a surface by ejecting the liquid through anorifice toward the surface without the tool making substantial contactwith the surface. The direct-write tool is preferably controllable overan x-y grid relative to the printed surface (i.e. either or both thesubstrate and device may move). One preferred direct-write toolaccording to the present invention is an ink-jet device. Other examplesof direct-write tools include automated syringes, such as the MICROPENtool (Ohmcraft, Inc. Honeoye Falls, N.Y.) and the DOTLINER dispensesystem (Manncorp, Huntingdon Valley, Pa.) which is capable of dispensinglines, dots and areas down to 200 μm or smaller at speeds of up to10,000 dots/hour.

According to the present invention, the orifice of the direct-write toolcan have a reduced diameter. This is a direct result of the particlecharacteristics discussed hereinabove. A reduced diameter will enablethe formation of finer features.

One preferred direct-write tool according to the present invention is anink-jet device. Ink-jet devices operate by generating droplets of inkand directing the droplets toward a surface. Ink-jet printing, whenapplied to the particulate suspensions in accordance with the presentinvention is a means for delivering controlled quantities of theparticles to a variety of substrates.

The position of the ink-jet head is carefully controlled and can behighly automated so that discrete patterns of the ink can be applied tothe surface. Ink-jet printers are capable of printing at a rate of 1000drops per second or higher and can print linear features with goodresolution at a rate of 10 cm/sec or more, up to about 1000 cm/sec. Eachdrop generated by the ink-jet head includes approximately 2 to 200picoliters of the liquid that is delivered to the surface. For these andother reasons, ink-jet devices are a highly desirable means fordepositing materials onto a surface.

Typically, an ink-jet device includes an ink-jet head with one or moreorifices having a diameter of less than about 100 μm, such as from about50 μm to 75 μm. Ink droplets are generated and are directed through theorifice toward the surface being printed. Ink-jet printers typicallyutilize a piezoelectric driven system to generate the droplets, althoughother variations are also used. Ink-jet devices are described in moredetail in, for example, U.S. Pat. No. 4,627,875 by Kobayashi et al. andU.S. Pat. No. 5,329,293 by Liker, each of which is incorporated hereinby reference in their entirety. However, such devices have primarilybeen used to deposit inks of soluble dyes.

Ideally, the droplet generated by the printer head is identical incomposition to the bulk fluid. However, some filtration of suspensionsmay occur if the particles are too large to pass through the channels oronboard filters. The small particle size and reduced number of particleagglomerates according to the present invention reduces the amount ofparticles collected by the filter and can enable removal of the filter.

According to the present invention, it is possible to deposit gradientlayers of material wherein the composition of the layer changes throughthe thickness of the layer. In order to deposit such layers, it ispreferred to form the layer using multiple direct-write deposition stepswherein the composition of the suspension being deposited changesthrough the layer. Such gradient layers are discussed in more detailbelow.

Utilizing the direct-write method of the present invention, it is alsopossible to form features and create device components on a non-planarsurface, if required for a specific application or product geometry.

According to the present invention, more than one type of particle canbe dispersed in a single liquid vehicle for deposition with adirect-write tool. The particles can be dispersed into the liquidvehicle by lightly mixing or, for example, by using ultrasound. For usein an ink-jet device, the viscosity of the suspension is preferably notgreater than about 30 centipoise, more preferably not greater than about20 centipoise. It is also important to control the surface tension ofthe liquid suspension and preferably the surface tension is from about20 to 25 dynes/cm for an ink-jet device.

The solids loading of fine particles in the suspension is preferably ashigh as possible without adversely affecting the viscosity or othernecessary properties of the direct-write composition. For example, thedirect-write composition can have a particle loading of up to about 75weight percent, such as from about 10 to about 50 weight percent.

The direct-write compositions are typically water-based, although othersolvents or liquids may be used. Such compositions can include otherchemicals including, but not limited to, surfactants, dispersion agents,defoamers, chelating agents, humectants and the like.

More specifically, ink-jet compositions generally include water and analcohol. Organic solvent based systems can also be used and ink-jetprint heads are often tailored for either organic or aqueous systems.Surfactants are also used to maintain the particles in suspension.Co-solvents, also known as humectants, are used to prevent the ink fromcrusting and clogging the orifice of the ink-jet head. Biocides can alsobe added to prevent bacterial growth over time. Examples of such ink-jetliquid vehicle compositions are disclosed in U.S. Pat. No. 5,853,470 byMartin et al.; U.S. Pat. No. 5,679,724 by Sacripante et al.; U.S. Pat.No. 5,725,647 by Carlson et al.; U.S. Pat. No. 4,877,451 by Winnik etal.; U.S. Pat. No. 5,837,045 by Johnson et al.; and U.S. Pat. No.5,837,041 by Bean et al. Each of the foregoing U.S. patents isincorporated by reference herein in their entirety. The selection ofsuch additives is based upon the desired properties of the composition,as is known to those skilled in the art. The fine particles are mixedwith the liquid vehicle using a mill or, for example, an ultrasonicprocessor.

According to one embodiment of the present invention, the liquid vehicleinto which the particles are dispersed includes soluble molecularprecursors, such as metal precursors, that have a relatively lowdecomposition temperature, as is illustrated in FIG. 38( b). Themolecular precursor is preferably a soluble inorganic compound that canbe co-deposited with the powders and then thermally treated to form anessentially continuous phase filling any void space between particles.Low temperature decomposition precursors such as those described hereinwith respect to spray drying can be used. A preferred type of precursorcompound are the alpha hydroxycarboxylate (glycolate) derivatives. Metalcarboxylates are often advantageous in this respect for the formation ofmetal compounds. It will be appreciated that the molecular precursorswill have a composition that is specific to the material beingdeposited. Ligands on the molecular precursors can act as a surfactantor the actual liquid vehicle.

In one embodiment, the molecular precursor forms essentially the samecompound as the particles. In this embodiment, the particles in theliquid vehicle can advantageously catalyze the molecular precursor toform the desired compound. The addition of precursors with decompositiontemperatures not greater than about 400° C., preferably not greater thanabout 300° C. and more preferably not greater than about 250° C. allowsthe formation of functional features on a polymeric substrate, includingpolyamide, fluoro-polymers, epoxy laminates and other substrates. Thesemolecular precursors are particularly useful when combined with hollowor porous particles because they contribute to higher densities when thedeposited layer is sintered. That is, a portion of the final layer comesfrom the particles and a portion from the molecular precursor whereinthe portion from the precursor fills in space between particles andthereby increases the solids fraction in the final structure.

The liquid vehicle can also include carriers to hold the particlestogether once the particles are deposited. Such a liquid vehicle wouldbe advantageous when the particles are to be deposited and will not besintered to adhere the particles to one another. The liquid vehiclecould also include a polymer that, after deposition, would yield apolymer layer with particles dispersed throughout the polymer. Further,the liquid vehicle could also include a molecular species which canreact with the dispersed particles to modify the properties of theparticles.

Other processes that can be utilized to fabricate the devices of thepresent invention include laser transfer and guided optical deposition.In a laser transfer method, a material that is to be deposited is placedonto a transfer substrate such as a glass disc or an organicpolymer-based ribbon. The transfer substrate is then placed over thesubstrate upon which the material is to be deposited. A laser is thenused to controllably transfer the material to the substrate from thetransfer substrate.

Guided optical deposition is a technique wherein the materials orprecursors to the materials are delivered through an optical fiber tothe substrate in a controlled manner such that features on the substratecan be formed by controlling the position of the optical fiber relativeto the substrate. Upon delivery of the material and or materialprecursor to the substrate, the material is heated if necessary toconvert the material or otherwise modify the material properties. Forexample, the material can be heated in a localized manner by using alaser.

Particles such as metal-carbon composite particles can also be depositedelectrophoretically or electrostatically. The particles are charged andare brought into contact with the substrate surface having localizedportions of opposite charge. The layer is typically lacquered to adherethe particles to the substrate. Shadow masks can be used to produce thedesired pattern on the substrate surface.

Patterns can also be formed by using an ink jet or small syringe(discussed above) to dispense sticky material onto a surface in apattern. Powder is then transferred to the sticky regions. This transfercan be done is several ways. A sheet covered with powder can be appliedto the surface with the sticky pattern. The powder sticks to the stickypattern and does not stick to the rest of the surface. A nozzle can beused to transfer powder directly to the sticky regions.

Many methods for directly depositing materials onto surfaces requireheating of the particles once deposited to sinter them together anddensify the layer. The densification can be assisted by including amolecular precursor to a material in the liquid containing theparticles. The particle/molecular precursor mixture can be directlywritten onto the surface using ink jet, micro-pen, and other liquiddispensing methods. This can be followed by heating in a furnace orheating using a localized energy source such as a laser. The heatingconverts the molecular precursor into the functional material containedin the particles thereby filling in the space between the particles withfunctional material.

A number of other methods may be employed to construct layers containingthe electrocatalyst powders according to the present invention. Forexample, the powders can be deposited by doctor blading, slot die orcurtain coater methods. In these methods, an ink or paste containing theelectrocatalyst powder is applied to the surface using a blade which isheld at a specified height from the substrate. The thickness of thelayer can be controlled down to several micrometers. For slot die andcurtain coater methods, the ink or paste is dispensed through a smallgap onto a substrate that may be moving on a web drive.

Roll pressing methods can also be used. Roll pressing methods involvemixing components including the electrocatalyst powder, binders andother property modifiers and feeding them through a roll mill to form apressed film. Roll pressing is often done directly on other active partsof the energy device such as a nickel mesh current collector.

Electrostatic printing methods can be used wherein the electrocatalystparticles are charged with an electric charge, transferred to the drumof a roller, then transferred to a substrate which has the oppositeelectric charge to that of the particles. This transfer can be carriedout in a fashion that results in a blanket layer over the entiresubstrate or in a patterned manner with the pattern determined by thedistribution of the electrical charge on the substrate surface.Typically this method enables the transfer of layers one particle thickand therefore enables very good control over layer thickness for thinlayers.

Gravure, rotogravure and intaglio printing methods can be used whereinan ink or paste containing the electrocatalyst powder is transferred toa sunken surface feature, often on a cylinder, that defines the patternto be transferred to the substrate surface. The substrate is often acontinuous feed from a web drive. Relief and flexographic printingmethods can also be used which are the reverse of Gravure printing inthat a material, often in the form of a paste or ink, is transferredfrom a raised pattern on a surface, often a roller, to a substrate.

Lithographic printing methods can also be used. In lithographic printingmethods, a photosensitive ink or paste is placed on the substrate andexposed to a source of illumination or electromagnetic radiation,generally UV light, wherein the exposed areas interact with thisradiation to undergo a change. The change may result in creation of asoluble or insoluble feature depending on the reactivity of the pasteand the desire for positive or negative lithography. After removal ofthe unwanted matter the patterned layer containing the electrocatalystpowder remains for further processing.

Laser transfer methods can be used in which the electrocatalystcontaining material is dispersed on a tape or ribbon and a laser is usedto transfer material from the underneath surface of the ribbon or tapeto the surface of the desired substrate which is close proximity to thetape. Using this method, features can be built with controlleddimensions.

Spray deposition methods can also be used. In spray deposition methods,an ink containing the electrocatalyst powder is fed through a spraynozzle and atomized to form droplets which are directed to a surfacewhere the electrocatalyst layer is to be deposited.

The fine powders produced according to the present invention result inthinner and smoother powder layers when deposited by such liquid or drypowder based deposition methods. Smoother powder layers are the resultof the smaller average particle size, spherical particle morphology andnarrower particle size distribution compared to powders produced byother methods.

The present invention is also directed to devices including thin filmprimary and secondary batteries and in one embodiment is directed tothin film air cathodes for use in such batteries. The thin film aircathodes are particularly useful in metal-air batteries such as Zn/Airprimary batteries and Zn/Air secondary batteries and novel batteriesreferred to herein as metal hydride/air (MH/Air) secondary batteries.The novel air cathode advantageously enables the reduction of oxygen(O₂) to hydroxyl ion (OH⁻) and the transport of the OH⁻ ions to theanode during discharge and transport O₂ to the liquid-solid interfaceduring discharge. For this reason, the thin film air cathodes of thepresent invention are also referred to as bi-functional oxygenelectrodes, since they combine both functions, namely oxygen reductionand oxygen evolution.

Metal-air batteries have the best potential for power density, peakpower characteristics, voltaic efficiency and rate capability among allbattery technologies. In addition, the components of a metal-air batteryare very suitable for printing to produce a light-weight, thin battery.The high rate of discharge is also advantageous for portable devicesthat require frequent high current discharge with a background of lowcurrent continuous operation.

The metal-air batteries according to the present invention includemultiple functional layers, two or more of which may be combined into asingle multi-functional layer. The functional layers can include amembrane layer, current collector, hydrophobic layer, electrocatalystlayer, an electrolyte, separator and anode.

The main electrocatalytic processes in the air cathode of a metal/airbattery take place in a 3-phase boundary (electrode/air/electrolyte)which is graphically illustrated in FIG. 39. The electrocatalyst foroxygen reduction must populate the zone of 3-phase contact 3502 and bein electrical contact with the electrode (current collector) 3504 and indiffusional contact with the electrolyte 3506 and the air 3508. Toaccomplish this, present metal air battery cathodes include agas-diffusion layer, a catalytic layer and a current collection system.The gas-diffusion layer is characterized by high gas permeability andimpermeability to aqueous solutions. The catalytic layer consists of aporous conductive matrix with a highly dispersed electrocatalyst toyield a distribution of hydrophobic pores for oxygen supply andhydrophilic pores for electrolyte exposure. The current collector isusually made from an inert metal mesh, such as nickel or nickel alloymesh in intimate mechanical contact with the pressed matrix of highlydispersed carbon.

It is desirable to maximize the exposure of the active electrocatalyticsites to both air and the electrolyte. According to the presentinvention, a gradient of hydrophilic/hydrophobic properties across thecatalytic layer in the zone of 3-phase contact can be utilized toenhance the properties of the air cathode. Various embodiments of theair cathode according to the present invention will now be describedwith particular reference to FIGS. 40–47.

FIG. 40 illustrates an air cathode 3600 according to one embodiment ofthe present invention. The air cathode illustrated in FIG. 40 canadvantageously utilize printing of the current collector 3602 andsequential printing of the electrocatalyst layer 3606 and carbonconductor layer 3604. The current collector 3602 is fabricated from aconductive metal such as nickel or silver and for many applicationssilver is preferred. The current collector 3602 can be deposited solelyfrom metal precursors or from metal precursors combined with dispersedmetal powders. The dispersed metal powders can be nanometer-sizedparticulate powders or can be high aspect ratio powders (e.g., fibers),such as fibers having an average length of 2 to 10 μm, which can providegood conductivity while being highly porous. The metal precursors shouldbe capable of decomposing into the metal at relatively low temperatures,such as not greater than about 400° C., more preferably not greater thanabout 250° C. For example, silver metal precursors can be chosen fromsilver carboxylates and silver trifluoroacetate, which can also includesilver nanoparticles. When silver nanoparticles are included in a silvertrifluoroacetate precursor, the thermal decomposition temperature can bereduced from about 350° C. to about 250° C. If the material is subjectedto a rapid thermal anneal or is laser processed, then it may be possibleto utilize higher temperature precursors due to the short exposure time.A thermally insulating layer, such as a porous aerogel layer, can alsobe used as a thermal insulator to reduce the thermal affects. Currentcollectors thinner than about 1 μm can be formed solely from the metalprecursors and will not require incorporation of metal powder, whilethose thicker than 1 μm will typically require the use of a metal powderprecursor.

The current collector 3602 must be deposited and processed at lowtemperatures onto a membrane gas diffusion layer 3603. The gas diffusionlayer 3603 is typically fabricated from TEFLON. TEFLON is atetrafluoroethylene (TFE) fluorocarbon polymer available from E. I.duPont deNemours, Wilmington, Del. Although the term TEFLON is used inthe present specification for convenience, it is understood that othersimilar fluorocarbon polymers can be substituted for TEFLON.

The current collector is preferably fabricated using a direct-writedeposition process. Advantageously, the current collector comprises aplurality of elongated strips having an average width of preferably notgreater than about 100 μm, such as not greater than about 75 μm. It willbe appreciated that the metal current collector can be fabricated byother methods, including sputtering, evaporation, photolithography,electroless plating, electroplating, doctor blade, screen printed orelectrochemical deposition.

A gas diffusion layer 3603 which allows maximum permeation of oxygen andno permeability to aqueous solutions using hydrophobic pores isnecessary as the pores of the gas diffusion layer need to be protectedfrom flooding by the electrolyte. This layer can be a continuous TEFLONmembrane or a pressed Teflonized carbon layer. For example, onepreferred TEFLON layer is about 90 μm thick with a density of 2.26cm³/g. The average pore size is about 23 nm, with a distribution ofpores ranging from about 0.2 nm to 70 nm, corresponding to a porosity ofabout 10% and a surface area of 7.3 m²/g.

In case of the Teflonized carbon (discussed below), the currentcollector is incorporated as a Ni mesh in the carbon with the metal meshbeing closer to the gas-open side. However in the case of the currentcollector being deposited directly on the TEFLON, the TEFLON surface ispreferably modified to promote adhesion between the current collectorand the TEFLON surface. Several routes can be utilized to modify thesurface of the TEFLON. A commonly used method to modify the TEFLONsurface is to etch the surface. Tetra-etch is a commonly used industrialetchant for TEFLON. Tetra-etch is a mixture of sodium naphthalene inethylene glycol dimethyl ether. The TFE TEFLON molecule is a long chainof carbon atoms to which fluorine atoms are bonded. The etchant stripsthe fluorine atoms from the chain creating a deficiency of electrons,which are then replaced with water vapor, oxygen, and hydrogen when theTEFLON is exposed to air. The carboxyl, carbonyl and hydroxyl groupsformed as a result of etching easily adhere the current collector on theTEFLON surface. Tetra-etch in the as received form is to strong to etchthe thin TEFLON layer and should be diluted for etching the TEFLONsurface.

Another approach to modify the TEFLON surface is to sputter a thin layerof metal film on the TEFLON surface. Examples of metals that can besputtered are Au and Cu. In one embodiment, a 40 nm Au layer issputtered on TEFLON, which enabled the Ag current collector to adhere tothe TEFLON. The characteristics of TEFLON were measured after surfacemodification of TEFLON and the etched TEFLON and TEFLON with a 40 nmthick Au sputtered layer retained their surface area and porosity whencompared to the unmodified TEFLON surface.

To deposit a conductive current collector 3602 it is often necessary toanneal the precursor to the conductive metal. Thus, it may be necessaryto anneal the TEFLON membrane in the further processing steps to make athin film battery. Thus, the effect of heat treatment on TEFLON wasinvestigated. Several strips of TEFLON were placed at differenttemperatures in a furnace for ten minutes. Since the glass transitiontemperature of TEFLON is 340° C. as measured from TGA/DTA data, thechanges in characteristics of TEFLON, if any, were measured at 100° C.,200° C., 250° C. and 300° C. There was a decrease from 7.3 m²/g to 5.9m²/g in surface area of the TEFLON on heating for ten minutes in afurnace at 300° C. A decrease in surface area is undesirable as itdirectly relates to the decrease in gas diffusion through this layer.There was no difference in surface area and porosity at temperaturesbelow 300° C.

An alternative to heating in a furnace is to use rapid thermalprocessing. Rapid thermal processing (RTP) is a versatile approach forseveral different processing functions, such as rapid thermal annealing(RTA), rapid thermal cleaning (RTC), and rapid thermal chemical vapordeposition (RTCVD). Rapid thermal systems are capable of increasingtemperatures in excess of 200° C./s. A rapid thermal process heats thematerial to a processing temperature by radiative heating.

TEFLON strips were annealed at 200° C., 250° C., 300° C., and 347° C.respectively for one minute each. Each of the TEFLON strips that weresubjected to RTP at the different measurement were then characterized interms of their surface area and porosity. A decrease in surface area to5.3 m²/g was observed when the TEFLON strip was subjected to RTP at 347°C. There was however no change in the surface area of the TEFLON below347° C. Thus, it is possible to subject TEFLON to RTP at highertemperatures than heating in a furnace. Table I illustrates thecharacteristics of TEFLON on annealing.

TABLE I Surface Pore Temperature Area Volume ° C. m²/g cm³/g TEFLON asreceived — 7.33 0.0489 In furnace for 10 minutes 100 7.81 0.0638 2007.51 0.0554 250 7.05 0.0724 300 5.91 0.0390 RTP for one minute 200 7.920.0625 250 8.04 0.0518 300 7.74 0.0544 347 5.38 0.0525 40 nm Au onTEFLON — 7.6 0.0568 Etched TEFLON — 7.6 0.0619

FIG. 41 illustrates a photomicrograph (40×) of a silver currentcollector deposited on an etched TEFLON membrane using a direct-writemethod. The silver precursor included silver trifluoroacetate and silvermetal nanoparticles. After deposition, the assembly was heated at 250°C. for 10 minutes to form the current collector. The average width ofthe current collector lines is about 75 μm.

Referring back to FIG. 40, the electrocatalyst 3608 is preferably anoxygen deficient Co—Ni—O metal oxide for secondary batteries andcomposite C/MnO₂ or C/Pt for primary batteries. To form theelectrocatalyst layer 3606, the electrocatalyst particles 3608 aredispersed in a hydrophilic matrix 3610 having lower hydrophobicity thanthe hydrophobic matrix 3614. The carbon conductor layer 3604 is requiredto provide conductivity between the current collector andelectrocatalyst layer 3606. In this layer, the carbon particles 3612 aredispersed in a hydrophobic matrix 3614. The separator 3616 preferablyconsists of a material that can be applied by a direct write method,however, screen print, doctor blade, or other approaches can also beused.

The hydrophobic matrix 3614 can include certain forms of carbon,fluorocarbon polymers such as TEFLON and other organic species.Hydrophilic layers can include metal oxide based materials such as acarbon electrocatalyst coated with metal oxide active phases. Some typesof carbon and some organic polymers derivatized with hydrophilicfunctional groups (e.g., polyesters, polyethylene oxides, polyethers,polyalcohols and polycarboxylates) can also be used.

To form the carbon conductor layer 3604 and the electrocatalyst layer3606 the carbon particles 3612 and electrocatalyst particles 3608,respectively, can be dispersed into liquid vehicles and printed ontoeach other with controlled thickness. The carbon particles and/orelectrocatalyst particles can be TEFLONIZED by coating with TEFLON toform the hydrophobic matrix and the hydrophobicity can be controlled byadjusting the ratio of TEFLON to the particles.

One advantage of the embodiment illustrated in FIG. 40 is that theoverall thickness is preferably not greater than about 100 μm (excludingthe separator 3616). This results in several improvements includingreduced diffusional resistance in these layers. The thickness of thecurrent collector 3602 is reduced resulting in a smaller volume thatcorresponds to higher volumetric and gravimetric energy density, inaddition to a higher drain rate. The drain rate is higher because oncethe kinetic limitation of the electrocatalyst is removed by using a moreeffective catalyst material, the next limitation on the catalyticconversion is the rate at which the species can diffuse between layers.Therefore, in this particular case (using a liquid electrolyte incontrast to a solid electrolyte) not only does the volumetric andgravimetric energy density increase due to a reduced mass and volume,but the diffusing species travel a shorter distance, resulting in ashorter transport time, hence a faster drain rate. This is an advantageover a Li-ion battery for example because even if a printed currentcollector is used, the diffusing species (Li ions) still diffuserelatively slowly through the metal oxide solid LiMnO_(x) spinelelectrolyte.

FIG. 42 illustrates an air cathode 3700 according to another embodimentof the present invention including a printed current collector 3702 anda gradient in the electrocatalyst concentration through layer 3705.Layers 3604 and 3606 (FIG. 40) are combined into a single gradient layer3705 (FIG. 42). The same current collector metals can be used as isdiscussed above with reference to FIG. 40. The carbon andelectrocatalyst layers are combined into a single gradient layer 3705wherein the portion contacting the current collector 3702 includes ahydrophobic matrix and the portion contacting the separator 3716includes a hydrophilic matrix, resulting in a significant reduction inelectrode thickness. The ratio of hydrophobic matrix to hydrophilicmatrix varies through the layer 3705 accordingly. The fabrication of agradient in composition in the electrocatalyst/conductor layer 3705requires printing sequential layers with varying compositions (e.g.,ratio of TEFLON to carbon particles) ranging in degree ofhydrophobicity, concentration of electrocatalyst particles 3708 andconcentration of carbon particles 3712, all of which lead toimprovements in performance. Thus, thin layers of different compositionscan be printed successively wherein the composition of each layer issystematically varied. This produces a tailored composition gradient andtherefore the desired property can be achieved. Alternatively, thecomposition of the precursor may be continuously varied and therepeating layers leads to a composition gradient.

One advantage is that the overall thickness is further reduced leadingto higher energy density. Preferably, the current collector and gradientlayer have a total average thickness of not greater than about 50 μm. Inaddition, the compositional gradient creates a larger 3-phase contactzone, also leading to better performance.

FIG. 43 illustrates another embodiment of an air cathode 3800 accordingto the present invention including an electrocatalyst particle layer3808 printed directly over a current collector 3802. The combinedfunctionality of several layers advantageously eliminates the carbonlayer and provides an even thinner electrode. The carbon that wasrequired for conductivity (FIGS. 40 and 42) is eliminated due to theintimate contact between the current collector 3802 and theelectrocatalyst particles 3808.

Several approaches can be used to deposit the electrocatalyst 3808 onthe current collector 3802. The electrocatalyst 3808 can be depositedusing a direct-write method or can be formed directly on the currentcollector 3802 by vapor phase deposition.

The thickness of the electrode (not including the separator 3816) ispreferably not greater than about 30 μm, compared to about 400 μm for aconventional structure. Thus, diffusional resistances are reducedresulting in better performance.

FIG. 44 illustrates an air cathode 3900 including a composite currentcollector/electrocatalyst 3903 according to another embodiment of thepresent invention. This structure combines the functions of the currentcollector and the electrocatalyst into a single porous conductiveprinted pattern 3903. No diffusion of oxygen is required through thelayer 3903.

In this embodiment, the electrocatalyst and current collector arecombined into a porous composite structure 3903 with controlled wettingto obtain the 3-phase interface. This is accomplished by combining thepre-formed electrocatalyst particles 3908 with precursors to the porousmetal that can include metal particles and metal precursors. Thermalprocessing at low temperature converts the metal precursor to the metal,joining the metal particles to form a porous layer 3903 containing theelectrocatalyst. Layer 3903 can be a metal ceramic composite such as asilver or nickel ink containing electrocatalyst particles such as aNiCoO_(x). In this case, a lower temperature route compatible with thesubstrate (e.g., porous fluorocarbon polymer) can be used. Otheradditives that aid in the decomposition of the silver precursor to formsilver such as reducing agents can be included. Silver pastes used inpolymer thick film applications may also be useful.

Further, composite particles such as metal/metal oxide particles can beuseful for this layer. For example, a metal or metal alloy such as Ag/Pdwith embedded perovskite metal oxides (e.g., MgTiO₃) can be useful.

In this embodiment, the electrons generated at the surface of theelectrocatalyst 3908 are captured directly by the current collector3902. This leads to better current collection efficiency, as well as afaster drain rate.

FIG. 45 illustrates an air cathode 4000 according to a furtherembodiment of the present invention wherein the cathode 4000 includes aporous composite current collector/electrocatalyst 4003. The compositelayer 4003 combines the current collector and electrocatalyst in acontinuous porous layer 4003 which also includes a hydrophobicitymodifier, such as a fluorocarbon polymer. An example is liquid TEFLON,an emulsion containing small TEFLON particles, or various modifiedfluorocarbon polymers. A TEFLON emulsion can be incorporated by one ofthe methods such as those described above. Oxygen is able to diffusethrough the porous layer, which is about 30 μm thick.

This composite layer approach relies on the mixing of several componentsincluding particles of a metal, TEFLON and electrocatalyst with othercomponents. The metal particles have a controlled particle sizedistribution. This leads to a well-controlled pore size distributionwherein the pore size is defined by the size of the spaces betweenparticles.

Various types of compositional gradients can be fabricated for thecomposite layer 4003. For example, a porosity gradient can be formedthrough control of the particle size distribution as a function oflocation in the layer. A hydrophobicity gradient can be formed byvarying the concentration of the TEFLON-type material. Theelectrocatalyst concentration can also be varied. Further, conductivitycan be varied by control of the metal particles and molecular metalprecursors.

Vapor infiltration can also be used to form various useful structuressuch as those discussed above. In this process, a bed of particles isfirst deposited using a direct write process. The bed is heated andexposed to a reactive vapor that carries out CVD or ALE to depositmetals or metal oxides. This vapor-infiltration method has severalpotential benefits including enhanced catalytic activity, the ability tofuse particles to each other, the ability to oxidize or reduce certainspecies, the ability to control site specific reactions, the ability todeposit MnO₂, silver, and other metals and metal oxides at lowtemperatures and the ability to modify the hydrophobicity of materialswith suitable silanating or similar agents.

For the construction of 3-dimensional layered devices, alternating“monolayers” of particles can be deposited that will formthree-dimensional architectures with considerable performanceimprovements. This approach will be most beneficial when alternatingmonolayers of metal particles as the current collector with monolayersof electrocatalyst particles. This 3-dimensional structure leads toperformance improvements as a result of the high surface area andintimate contact between conductor and electrocatalyst particles. Thisdesign is schematically illustrated in FIG. 46.

In the embodiment illustrated in FIG. 46, the device 4100 can befabricated as follows. The base 4102 (gas diffusion layer) is coatedwith a composite layer 4103, preferably using a direct-write method.This can be done with multiple jets/heads in series with differentcompositions in each to form a quasi-gradient. The layer 4103 includesalternating thin layers of current collector particles (4105, 4106,4107, 4108) and electrocatalyst particles (4109, 4110 and 4111). Theparticle layers are dispersed in a hydrophobic matrix near the base 4102and a hydrophilic matrix near the separator 4116. Thehydrophobic/hydrophilic ratio changes accordingly through the thicknessof the layer 4103. Then an overcoat of electrolyte composition isapplied using similar methods or other technologies. For example, theelectrolyte can be an aqueous solution of potassium hydroxide, KOH. Itcan be deposited as part of the ink formulation throughout the printedlayer in which case an additional overcoat may not be necessary. Thelayers can also be deposited without the electrolyte, which can then beapplied as an overcoat afterwards to infiltrate the underlying layerswhen it can be deposited using a method that can withstand the corrosionof the KOH. A separator layer 4116 is then applied using a direct-writemethod.

It is expected that when decreasing feature size and layer thickness inthe air cathode there will be a point at which further reduction in sizewill be detrimental to battery performance. It is possible to printlayers that are about one particle thick which corresponds to dimensionsof about 1 to 2 μm. At these sizes it is possible that certainparameters such as pH, concentration, and electric field gradients maydominate the performance of the device and possibly be detrimental. Thelayer in which this is likely to have the most significant effect is inthe current collector. The line width and pitch can be varied from theextreme of a largely “transparent” grid to a microporous layer thatcould limit battery performance due to a large IR drop. Calculationsindicate that down to a layer thickness and feature size of 20 μm, thereis no significant problem of IR drop.

One of the problems associated with batteries that use electrolytes iscarbonate formation from CO₂. A CO₂ reduction layer can be used toalleviate this problem. For example, selective adsorption of CO₂ by ahigh surface area metal oxide such as Group II metal oxide can be used.The molar volume increase on formation of MCO₃ from MO on reaction withCO₂ is considerable which may result in restricted mass transport of O₂in the cell depending on the porosity and other factors. Therefore,heavy metal oxides are preferred since the expansion in volume decreaseswith increasing atomic weight of the metal ion. As an alternative, thelayer can be used to initiate a catalytic reaction to convert the CO₂ toan inert or even useful species. This can have the additional advantagethat oxygen is formed which can benefit cell performance. This layermust be placed between the air and the electrocatalyst layer. FIGS. 47(a) and 47(b) illustrate two placements for this layer. In FIG. 47( a)the CO₂ reduction layer 4218 a is placed between the electrocatalystlayer 4206 a and the carbon conductor layer 4204 a. In the embodimentillustrated in FIG. 47( b), the CO₂ reduction layer 4218 b is placedbetween the base 4201 b and the current collector 4202 b.

The fine particles and methods for depositing the fine particlesaccording to the present invention are useful for the fabrication of anumber of novel devices such as primary and secondary batteries. Suchdevices are included within the scope of the present invention.

For example, the thin film air cathodes of the present invention anddescribed above are particularly advantageous for use in the electrodesof rechargeable batteries such as rechargeable zinc-air batteries. Azinc-air battery is schematically illustrated in FIGS. 48( a) and (b).

Specifically, FIG. 48( a) illustrates a zinc-air battery 500 in chargingmode. The battery 500 includes air electrodes (cathodes) 502 and 508 anda zinc electrode (anode) 504 which includes a layer of zinc 506. Theelectrodes are typically packaged in a flat container that is open tothe air. When the battery cell discharges, the zinc metal 506 isoxidized to Zn²⁺. When all the zinc has been oxidized, the battery 500is recharged and Zn²⁺is reduced back to zinc metal 506. The direct-writedeposition methods of the present invention can advantageously be usedto produce such electrocatalytic devices by depositing the metal-carboncomposite powders in discrete patterns, having a thin, dense structure.

The present invention is also directed to a novel battery system that isa hybrid of existing metal hydride and zinc/air technologies, referredto as a metal hydride/air (MH/Air) battery. The properties of differentbattery systems are illustrated in Table II.

TABLE II Battery Specific Energy Energy Density Specific Power CycleSystem (Wh/kg) (Wh/L) (W/kg) Life Li-ion 250 200 100–200 1000 Metal 70250  70–280 500 Hydride Zinc/Air 250 200 200–450 200 Metal 320 250100–350 1000 Hydride/Air

The metal hydride/air battery according to the present inventionadvantageously combines the advantages of the anode from a metal hydridebattery with the air cathode of the present invention. As is illustratedin Table II, the metal hydride/air battery provides many of theadvantages of a zinc/air battery such as high specific energy andspecific power, but also has an increased cycle life.

The metal hydride/air battery according to the present inventionincludes a metal hydride anode and an air cathode, with an alkalineelectrolyte disposed between the two electrodes. During discharge,oxygen and water are converted to hydroxyl ions which are transported tothe anode where they react with the metal hydride to form electronswhich can be routed to produce energy. During recharge, the water isreacted at the metal hydride electrode to create hydroxyl ions which arethen reacted at the oxygen electrode to liberate oxygen.

The metal hydride/air batteries of the present invention areparticularly useful in miniaturized devices such as GPS (GlobalPositioning System) transceivers. Each metal hydride/air battery cellcan provide approximately 0.9 volts of power and at least four suchcells would be utilized in a GPS battery to provide a total voltage of3.6 volts, which is sufficient for GPS requirements. The battery isthin, light-weight and can be recharged many times. It is estimated thateach cell would have a mass of about 4 grams. Although the battery has aslightly lower power density than a zinc air battery, the battery has amuch longer useful life. The air cathode which permits recharge can becombined with different anodes to tailor the performance for differentapplications. Such applications can include, but are not limited tounmanned vehicles, smart cards, GPS transceivers, RF tags, varioussensors, immunoassays, telemetry and other portable communications.

FIG. 49 schematically illustrates a metal-air battery in discharge mode.FIG. 50 schematically illustrates a metal air battery in charging mode.Metal-air rechargeable batteries were previously limited by problemswith the air electrode. The problems included rechargeability, cyclelife and environmental stability.

The direct-write deposition process of the present invention enableshigh performance battery such as the foregoing to be fabricated. Themethod is adaptable to different performance requirements, produces thinand light weight layers, is cost effective and efficiently uses thematerials. The ability to digitally control the deposition allows simpledesign changes to be made.

The batteries advantageously provide improved volumetric and gravimetricenergy density, increased capacity, increased cycle life, higherdischarge rate and a wide temperature range of operation.

The present invention is also applicable to a number of differentbattery technologies. For example, the methodology can advantageously beapplied to the production of prismatic batteries. The technology canalso be applied to supercapacitors to provide peak power for specificapplications. The methodology of the present invention advantageouslyenables an increase in the number of recharge cycles, increase powerdensity, increase specific power, reduce layer thickness and reduce cellthickness thereby resulting in a smaller device.

The electrocatalyst powders of the present invention are also useful infuel cells. FIG. 51 illustrates a schematic cross section of a membraneelectrode assembly for a fuel cell according to an embodiment of thepresent invention. The membrane electrode assembly 550 comprises ananode 552 and cathode 554 which are typically constructed from carboncloth. The anode 552 and cathode 554 sandwich a catalyst layer 556 and558 on each side of a proton exchange membrane 560 which can befabricated from a material such as Nafion 117. Power is generated whenhydrogen is fed into the anode 552 through anode channels 562 andoxygen, such as from air, is fed into the cathode 554 through cathodechannels 564. In a reaction catalyzed by a metal catalyst in thecatalyst layers 556 and 558, the hydrogen ionizes to form protons andelectrons. The protons are transported through the proton exchangemembrane 560 to the other catalyst layer where another catalystcatalyzes the reaction of protons with air to form water. The electronsare routed from the anode to the cathode via an electrical circuit whichprovides electrical power.

A class of fuel cell reactions that is required to be catalyzed is thereaction of a fuel such as hydrogen gas (H₂) to form H⁺ where, in thecase of a PEM fuel cell, the H⁺ is transported through a H⁺ iontransport membrane to the cathode. In this case, the fuel cell generallyoperates in acidic media and the cathode reduces O₂ to ultimately formwater as the final product. Other fuels may also be employed such asmethanol, natural gas or other hydrocarbons such as methane. In some ofthese cases other gases which may poison the reaction or catalyticallyactive sites such as CO are also present. These gases must be removed bythe presence of an alternative active composition to that which oxidizesthe fuel. As a result, the electrocatalysts aid in the removal orconversion of such species to benign products. Examples of such fuelcells are PEM and phosphoric acid fuel cells.

In some cases, catalysts are also required to convert the-feedstock fuelsuch as natural gas to a reactant having a higher H₂ content. Thisimproves the efficiency of the fuel cell and reduces formation ofcatalyst poisons. The catalytic compositions of the present inventionare also useful to catalyze this reaction.

EXAMPLES

1. MnO_(x)/C Composite Particles

Two groups of MnO_(x)/C composite electrocatalyst examples were preparedaccording to the present invention. The first group, described in TableIII, was prepared by ultrasonic aerosol generation and heating theaerosol in a hot-wall reactor (tubular furnace). The second group,described in Table IV, was prepared using a spray nozzle to generate anaerosol which was heated in a spray dryer. Air was used for the carriergas for all examples.

TABLE III Experimental conditions for ultrasonically generatedelectrocatalysts Additional Reactor Precursor Example Mn Surfactant TempMn Concentration Number precursor (wt. %) (° C.) (wt. %) (wt. %) 19AKMnO4 None 400 10 5 19B Mn nitrate None 350 10 5 20A KMnO4 None 350 10 520B KMnO4 None 350 10 5 23A KMnO4 0.017 350 10 5 24A KMnO4 0.034 350 105 27A Mn nitrate 0.049 350 10 5 28B KMnO4 0.049 300 10 5 28D KMnO4 0.049250 10 5 28E KMnO4 0.049 200 10 5 29B Mn nitrate/ 0.012 350 10 5 KMnO4

TABLE IV Experimental conditions for spray nozzle generatedelectrocatalysts Additional Reactor Precursor Example Mn Surfactant TempMn Concentration Number precursor (wt. %) (° C.) (wt. %) (wt. %) 30AKMnO4 0.078 208 10 5 30C KMnO4 0.078 208 10 5 34B KMnO4 0.078 208 10 541A KMnO4 0.083300 315 10 5 41B KMnO4 0.006700 315 10 5 41C KMnO40.083300 315 20 5 41D KMnO4 0.083300 315 10 5 44B None 0.000000 208 — 544C KMnO4 0.001600 208  5 5 44D KMnO4 0.001600 149  5 5 44E KMnO40.001600 149 10 5 44F KMnO4 0.001600 208 10 5 44G KMnO4 0.001600 208 105 47A None 0.000000 208 — 10 47B None 0.000000 208 — 5 47C None 0.000000208 — 2.5 47D KMnO4 0.000000 208 10 2.5 47E KMnO4 0.001600 208 10 2.07

The carbon precursor for all examples listed in Tables III and IV wasGRAFO1300 (Fuchs Lubricant Co., Harvey, Ill.) an aqueous dispersion ofparticulate carbon which has an average particle size of about 30nanometers and a surface area of about 254 m²/g. The aqueous dispersionalso includes an anionic surfactant. Additional amounts of a nonionicsurfactant (TRITON X-405, Sigma-Aldrich, St. Louis, Mo.) were added insome of the examples as is indicated in Tables III and IV. Triton X-405is a 70 wt. % solution of polyoxyethylene(40)-isooctylphenylether inwater. The GRAFO 1300 was suspended in water and the Mn precursor,previously dissolved in water, was slowly added to the carbon suspensionwhile stirring. The surfactant, added to the carbon suspension prior tothe Mn precursor, reduces precipitation when the Mn precursor is added.The reaction temperature for all examples was maintained below about400° C. since excessive temperatures (e.g., above 600° C.) can burn-offcarbon when air is used as the carrier gas.

More specifically, for the spray nozzle generation (Table IV), a batchof MnO_(x)/C powder was prepared in a spray drying apparatus in thefollowing manner. 35.6 kg (78.3 lbs) of carbon paste was added to abatching vessel. 65 kg (143 lbs) of de-ionized water was then added tothe carbon paste and mixed thoroughly. 0.13 kg (0.286 lbs) of thenonionic surfactant was added to the mixture and the mixture was stirredfor approximately 10 minutes. In a separate vessel, 2.27 kg (5 lbs) ofpotassium permanganate was dissolved in 65 kg (143 lbs) of de-ionizedwater. The solution was mixed for 20 minutes to allow the KMnO₄ todissolve. The KMnO₄ solution was then slowly added to the carbon paste.

FIG. 52 illustrates a scanning electron microscope (SEM) photomicrographof Example 23A (Table III). The particle morphology illustrated in FIG.52 is typical for the ultrasonically generated samples. The particleshave a spherical shape with the particle size varying between about 0.3μm and 10 μm. The support phase consists of primary carbon particles. Asis illustrated in the transmission electron microscope (TEM)photomicrograph image of FIG. 53, the support phase has a porousstructure.

The particles can be partially dissociated to smaller aggregates bymechanical force, such as ultrasound or compressing the particles byrolling into a layer. FIGS. 54 and 45 illustrate the ultrasonicallyinduced dissociation for a typical electrocatalyst powder produced byultrasonic generation. FIG. 54 illustrates the size distribution beforebreaking up secondary particles by sonification and FIG. 55 illustratesthe same powder after sonification.

FIG. 56 illustrates an SEM photomicrograph of Example 30C, generated bya spray nozzle. A comparison of FIGS. 52 and 56 illustrates that thespray generation method has a significant impact on the morphology ofthe secondary particles in terms of both shape and size. Some of thelarge particles produced by the spray nozzle deviate from the sphericalshape and have a “donut” shape.

FIG. 57 illustrates the size distribution of a spray dried powder. Thesecondary particles are larger, with diameters up to 20 μm, but thepowder has an average particle size of about 5 μm. The differences inthe secondary particles are related to the droplet size typical for thetwo aerosol generation approaches.

BET nitrogen absorption was used to measure the surface area andporosity of the electrocatalyst powders generated ultrasonically and bya spray nozzle. The results are summarized in Table V. If theultrasonically generated samples are compared, it is clear that theconversion temperature has an effect on the surface area. Example 19A,converted at 400° C., has a surface area of 93 m²/g, while Example 19B,converted at 350° C. has a surface area of 37 m² μg. However, furtherreduction in the temperature to 300° C. and 250° C. did not produce asignificant decrease in the catalyst surface area.

It also appears that the presence of surfactant has an impact on thesurface area. At identical conversion temperatures, the sample, whichhad additional amounts of surfactant in the precursor solution (Example29B) has a 40% lower surface area than the same powder with noadditional surfactant (Example 19B).

TABLE V Surface area of electrocatalysts BET Generation Surface AveragePore Example Method Area (m²/g) Diameter (nm) 19A Ultrasonic 93 — 19BUltrasonic 37 — 28B Ultrasonic 19 — 28D Ultrasonic 24 — 28E Ultrasonic19 — 29B Ultrasonic 25 — 34B Ultrasonic 21 — 41A spray nozzle 21 — 41Bspray nozzle 21 — 41C spray nozzle 17 — 41D spray nozzle 22 — 44C spraynozzle 28 20 44D spray nozzle 29 19 44E spray nozzle 24 17 44F spraynozzle 24 20 44G spray nozzle 24 21 44H spray nozzle 36  9 47A spraynozzle 36 23 47B spray nozzle 36 23 47C spray nozzle 33 23 47D spraynozzle 24 18 47E spray nozzle 25 16

The pure carbon samples (no surfactant) have the highest surface area ofabout 35 m²/g. The presence of precursors to MnO_(x) in the solutiontherefore leads to a reduced surface area and the surface areas forMn-containing samples are on the order of 20 to 25 m²/g, which iscomparable to the surface areas of the ultrasonically generated samplesat similar conversion temperatures.

Therefore, the selected aerosol generation method primarily impacts theparticle size distribution, while the conversion temperature primarilyimpacts the surface area of the MnO_(x)/C particles. However, the effectof conversion temperature on the surface area at temperatures below 300°C. is minimal. No significant changes were observed in the pore sizedistribution for the catalysts as a function of the preparationconditions. For all spray nozzle generated samples the average pore sizewas on the order of 20 nanometers, which indicates a secondary carbonsupport phase with no significant micro-porosity.

XPS analysis was also performed on the MnO_(x)/C powders. XPS (X-rayPhotoelectron Spectroscopy) analysis provides information about thesurface composition and Mn oxidation state for the electrocatalysts.Three characteristics of the XPS spectra were analyzed for comparison ofsamples generated under different conditions:

1) Positions of the binding energy of Mn 2p_(3/2) photoelectrons whichare indicative of the Mn oxidation state;

2) Relative intensities of Mn 2p_(3/2) vs. C 1s photoelectron peaks,directly compared between samples, for an indication of MnO_(x)dispersion or for model-based calculations of the average particle sizeof the dispersed MnO_(x); and

3) Ratios between different binding energies of O 1s photoelectron peaksrelated to: O₁ bonded to the C support, O₂ bonded to Mn and O₃ bonded inthe surfactant used in the precursor formulations.

Two commercial catalysts were evaluated for comparison to the powders ofthe present invention. Each were standard MnO_(x)/C powders availablefrom commercial manufacturers and used in zinc-air battery applications.Three standards were also analyzed to identify the Mn oxidation state inthe electrocatalysts. (MnO₂ powder, Mn₂O₃ powder and KMnO₄ powder.)

The preparation conditions, recording of the spectra and data processingwere identical for all samples. The samples were prepared for XPSanalysis by pressing the powder into indium (In) foil (99.9%),previously cleaned in HNO₃ to remove C and O impurities at the surface.

The XPS spectra for three control samples (Mn₂O₃, MnO₂ and KMnO₄) andall electrocatalyst powders were recorded on an AXIS HSi (KratosAnalytical) spectrometer, working in ΔE=constant mode at a pass energyof 80 eV, using an aluminum anode (Al K_(α)=1486.7 eV, 225 W). Theresidual pressure in the analysis chamber was 1×10⁻⁹ Torr. The peakpositions were estimated relative to the binding energy of C 1s=284.6eV. The following XPS peaks, designated by their electron levels, wererecorded: Mn 2p, C 1s, O 1s and K 2p. One survey scan was acquired inthe 75–1175 eV binding energy range for the control samples andelectrocatalyst powders before the high resolution spectra wereacquired. The experimental intensities were estimated from the areas ofthe corresponding peaks, measured on smoothed original peaks. The peakareas of Mn 2p and K 2p peaks include the areas of both 2p_(3/2) and2p_(1/2) peaks.

Spectra were obtained for control samples and the results areillustrated in Table VI. The Mn 2P_(3/2) peak in the XPS spectra ofKMnO₄ consists of two peaks and therefore two different oxidation statesof Mn are present.

TABLE VI XPS data for control samples Mn 2p_(3/2) O 1s K 2p_(3/2) C 1sSample (eV) (eV) (eV) (eV) Mn₂O₃ 641.6 529.6 — 284.6 MnO₂ 641.8 529.4 —284.6 KMnO₄ 641.5 529.8 290.8 284.6 644.0 293.0

The literature data on the Mn 2p_(3/2) binding energy show that itdepends on the oxidation state of Mn as follows:

Mn (II)—in MnO −640.6 eV

Mn (III)—in Mn₂O₃ −641.6 eV

Mn (IV)—in MnO₂ −642.6 eV

An increase of the XPS binding energy with increasing oxidation state ofthe element is a generally observed trend for a variety of materials.Since the oxidation state of Mn in Mn₂O₃ is Mn (III) and in MnO₂ is Mn(IV), it is expected that the binding energy for the latter should behigher than for the former compound. However, the experimental data showidentical binding energy (within the experimental error) for Mn 2p_(3/2)photoelectrons in Mn₂O₃ and MnO₂ control samples.

However, since MnO₂ is a strong oxidizing agent, it is not surprisingthat the average oxidation state of Mn near the surface is less than Mn(IV). Another possible reason is that X-ray induced reduction takesplace for the MnO₂ powder under the X-ray beam exposure.

The foregoing XPS results suggest that Mn (III) and Mn (IV) oxidationstates cannot be clearly distinguished. Still, a general trend forhigher binding energy indicates a higher Mn oxidation state.

Table VII contains information on the binding energy of the Mn 2p_(3/2′)O 1s and C 1s for two commercial electrocatalyst samples, samples 1A and2A.

TABLE VII XPS data for commercial electrocatalyst samples Mn 2p_(3/2)peak K 2p_(3/2) peak O 1s peak position position Position Sample (eV)(eV) (eV) 1A 642.3 292.2 529.7–57% 532.1–43% 2A 642.4 292.7 529.8–62%531.9–35% 534.6–3%

The comparison of the Mn 2p_(3/2) binding energy, 642.3 eV for Sample 1Aand 642.4 eV for Sample 2A does not show any significant difference.This binding energy is 0.7 eV higher than the binding energy of 641.6 eVobserved for Mn (III) in the Mn₂O₃ control compound. The Mn 2p_(3/2)binding energy observed for both commercial electrocatalysts is veryclose to the position of 642.6 eV, which according to the literaturedata corresponds to Mn (IV) oxidation state. It is highly probable thatX-ray induced reduction effect, observed for the MnO₂ control sample, isless expressed or not at all present for the electrocatalyst samples. Itcan be speculated that the MnO₂ species which are highly dispersed andin close contact with the conductive carbon surface are less likely toundergo an X-ray induced reduction than the highly crystalline MnO₂compound.

Therefore, the average Mn oxidation state in the commercialelectrocatalysts is between Mn (III) and Mn (IV) and most probably is Mn(IV). The Mn 2p_(3/2) binding energy position can be used as a referencefor achieving Mn oxidation state favorable for electrocatalytic activitywhen evaluating the electrocatalysts according to the present invention.The XPS measured Mn oxidation state may be slightly different from theoxidation state in actual conditions.

Table VIII contains a summary of the XPS data for electrocatalystExamples 19A through 34B (Tables III and IV). XPS data for Example 30D(Example 30A further heated to 250° C. in air for 1 hour) and Example33A (Example 30A further heated to 170° C. in air for 1 hour), are alsoincluded.

TABLE VIII XPS data for electrocatalyst samples Mn 2p_(3/2) peak K2p_(3/2) peak O 1s peak position position position Sample (eV) (eV) (eV)19A 642.4 292.6 529.9–59% 531.3–32% 533.0–9% 19B 642.0 293.0 529.8–21%532.2–79% 23A 642.3 292.8 528.9–59% 532.2–41% 28D 642.3 292.8 530.0–30%531.3–26% 532.9–44% 28E 642.1 293.1 529.9–22% 531.3–42% 533.1–36% 29B641.4 292.9 — 30A 642.6 293.0 528.9–21% 532.2–79% 30C 642.2 292.8528.9–21% 532.2–79% 30D 641.5 292.9 — 33A 641.3 293.0 — 34B 642.3 293.0—

The Mn 2p_(3/2) peak position for the samples obtained with Mn(NO₃)₂ asa precursor (Examples 19B and 29B), are lower by 0.4–1.0 eV compared tothe commercial electrocatalysts. The Mn 2p_(3/2) binding energy for themajority of electrocatalysts, 19A, 23A, 28D, 30A, 30C and 34B is similarto the position for the commercial samples. This result indicates thatthe Mn oxidation state in these samples is Mn (IV), while the Mnoxidation state in Examples 19B and 29B is closer to Mn (III), becausethe binding energy value is similar to that for the Mn₂O₃ controlsample. Since Examples 19B and 29B originate from precursor formulationscontaining Mn(NO₃)₂ as opposed to KMnO₄, it is clear that differentprecursor formulations result in different MnO_(x) surface species inthe electrocatalysts, and therefore different catalytic activity. Anaverage oxidation state close to Mn (IV) is likely most beneficial forthe electrocatalytic activity of the samples as is discussed with theelectrocatalytic activity data hereinbelow.

For Examples 30D and 33A, there is a shift of about 1.0 eV to a lower Mn2p_(3/2) binding energy compared to their corresponding counterpartbefore the heat treatment (Example 30A). This is an indication that thepost heat treatment leads to a reduction of Mn (IV) to Mn (III)oxidation state and therefore may be undesirable.

The O 1s spectra for Example 19A is characteristic of examples where theconversion of the precursor is complete. Therefore, the ratio of thedifferent O 1s photoelectron peaks for each sample can be used toestimate the ratio of MnO_(x) crystallite surface and the carbon supportsurface which is not covered by MnO_(x) crystallites. Only 9% of the O1s peak intensity can be related to the presence of oxygen fromsurfactant which was not reacted during the spray conversion. For theother limiting case, Example 19B, the O 1s peak at 532.2 eV, whichaccounts for about 80% of the O 1s peak intensity, corresponds to O inNO₃ species and its presence indicates non-complete conversion of theprecursor. Example 19B demonstrates significantly lower electrocatalyticactivity compared to Example 19A, as is discussed below.

The only difference in the preparation conditions between Examples 19A,28D and 28E is the spray conversion temperature. Comparing the O 1sregion, it is clear that while for Example 19A (400° C.) there is nosignificant O 1s peak associated with the presence of a surfactant, forExamples 28D (250° C.) and 28E (200° C.) that peak (533.2 eV) accountsfor 30–40% of the O 1s intensity.

Therefore, the spray conversion temperature influences the presence ofsurfactant in the catalyst powders. Since the remaining surfactant isdeposited either on top of the active MnO_(x) species or on the carbonsurface, it could potentially influence the catalytic activity of thesamples. Therefore, in order to minimize eventual negative effect of thesurfactant, either higher conversion temperatures should be used or thepresence of surfactant in the spray solution should be minimized.

The XPS data also contain information on the dispersion of the MnO_(x)species on the carbon support surface. This information is indirectlyincluded in the relative intensities of I (Mn 2p)/I(C 1s). In order toextract the information on the dispersion, several other parameters areneeded for the electrocatalysts such as the bulk composition of thesamples, the BET surface area and the theoretically calculated relativeintensities for monolayer distribution.

Table IX contains information about the bulk composition of the samplesanalyzed both by Atomic Absorption Spectroscopy (AAS) and X-rayFluorescence (XRF). XRF data generally show higher values for the Mn andK compared to the AAS data. The results suggest that the electrocatalystpowders of the present invention have higher molar concentration of bothMn and K than the commercial samples. Absolute values for the weightpercent concentration by AAS for the electrocatalysts of the presentinvention closely match the expected values, based on the composition ofthe precursor solution.

TABLE IX AAS/XRF data for the bulk composition Sample Mn/C K/C Or Mn Kat. ratio at. ratio Example (wt. %) (wt. %) x10² x10²  1A 1.78/2.800.90/1.47 0.40/0.64 0.28/0.48  2A 6.00/9.54 2.67/3.87 1.44/2.320.90/1.37 19A 9.04/14.8 7.94/12.2 2.38/4.43 2.93/5.13 19B 9.06/14.92.48/3.70 2.24/4.00 0.86/1.40

Table X contains data on the BET surface area, the theoreticallycalculated values for I (Mn 2p)/I(C 1s) relative intensities if theMnO_(x) species were to be distributed as a monolayer and the I (Mn2p)/I(C 1s) experimentally measured values.

Commercial Sample 2A includes a high-surface area activated carbon. Allof the electrocatalyst powders of the present invention have an order ofmagnitude lower surface area, formed after the primary high surface areacarbon support forms the secondary carbon support structures asdescribed above. The high-surface area activated carbon support (Samples1A and 2A) possess a significant degree of internal microporosity, whilethe spray converted secondary support formed in accordance with theforegoing examples has primarily mesoporosity.

TABLE X XPS modeling data for MnO_(x) particle size estimation Voltage(V) at I (Mn 2p)/ I (Mn 2p)/ Surface Estimated discharge Sample/ I (C1s) I (C 1s) area particle size current of Example (Experimental)(monolayer) (m²/g) (nm) 300 mA/cm² 1A 0.378 0.205 121  Non- —homogeneous distribution 2A 2.803 0.340 713  Non- 0.82 homogeneousdistribution 23A 1.188 1.754 93  2 1.02 30A 0.863 6.651 24 15 0.91 30C1.090 6.651 25 12 0.95 30D 0.688 6.650 30 40 0.76

The information for the estimated average MnO_(x) particle size isextracted by a comparison between the theoretical and experimentalvalues for 1 (Mn 2p)/I(C 1s) relative intensities. The changes in XPSrelative intensities and the comparison of the experimental data to thetheoretical ones is based on the method of Kerkhof and Moulijn (F. P. J.M. Kerkhof and J. A. Moulijn, J. Phys. Chem. 83, (1979)1612). Thisapproach as previously applied to dispersed catalysts (P. Atanasova andT. Halachev, Applied Catalysis A: General 108 (1994) 123; P. Atanasovaet al., Applied Catalysis A: General 161 (1997) 105) provides reliableinformation about the distribution of the active components on adispersed support. When the experimental value for I (Mn 2p)/I(C 1s)relative intensities is close to but lower than theoretical, an estimateof the particle size is possible through this XPS model. When theexperimental value for I (Mn 2p)/I(C 1s) relative intensities is higherthan theoretical, no exact estimate of the particle size is possiblethrough this XPS model. However, this is an indication for anon-homogeneous distribution of MnO_(x) species preferentially on theexternal surface area of the carbon support.

For Samples 1A and 2A, the experimental values for the I (Mn 2p)/I(C 1s)relative intensities are significantly higher than theoretical.Therefore, the total surface area of the carbon support is noteffectively utilized and the MnO_(x) active species are localized mainlyon the external surface area of the carbon support. The result is notsurprising since the activated carbon surface area includes asignificant degree of microporosity. During the wet processing used toform such powders, this porosity is not accessible for adsorption ofprecursors from the liquid phase due to wetting characteristics.

In contrast, the experimental values for the I (Mn 2p)/I(C 1s) relativeintensities for the electrocatalysts of the present invention are lowerthan theoretical and from the deviation an average MnO_(x) particle sizewas estimated for each sample, as is detailed in Table X. The estimatedaverage particle size varies from 2 nanometers for Example 23A to 40nanometers for Example 30D. The result for Example 23A indicates uniformdeposition of the active species throughout the carbon support surfacearea and only a few monolayers of MnO_(x) surface species. It isapparent that the dispersion varies depending on the preparationconditions, the relevant parameters being the type of spray generationand the spray conversion temperature. However, the XPS modeling datademonstrate uniform deposition throughout the carbon support surfacearea.

Table X also includes information on the electrocatalytic testing. Thevoltage attained by an electrode prepared with the electrocatalysts ofthe present invention in half-cell experiments at a discharge current of300 mA/cm² was chosen as a parameter for comparison of theelectrocatalytic activity of the catalysts.

FIG. 58 illustrates the correlation between the electrocatalyticactivity and XPS estimated average particle size from the data in TableX. There is a linear correlation between the electrocatalyticperformance of the catalysts and the average MnO_(x) crystallite size.It is important to note that all catalysts compared in FIG. 58 (exceptExample 30D) have identical Mn 2p_(3/2) binding energy, indicating anidentical Mn oxidation state. Based on the XPS model, no estimation ofthe MnO_(x) cluster size was possible for the commercial catalysts sincethey had a non-homogeneous distribution wherein MnO_(x) waspreferentially deposited on the outer support surface. However, if theelectrocatalytic performance for the Sample 2A catalyst is compared tothe data in FIG. 58, the corresponding MnO_(x) size for thiselectrocatalyst is about 30 nanometers.

The combined information on the Mn oxidation state and MnO_(x)dispersion derived from the XPS analysis is a valuable source forclarifying the MnO_(x)/C electrocatalyst structure and for predictingthe electrocatalyst performance. Achieving a Mn oxidation state that isoptimal for the electrocatalytic performance is probably the mostcritical requirement. However, forming the active species in a highlydispersed form is also important. The higher the dispersion, the higherthe number of active centers exposed to the electrochemical reagents andcatalyzing the reaction.

In order to confirm the XPS estimated average crystallite size, severalother analytical techniques were used. An X-Ray Diffraction (XRD)spectrum of a control sample prepared by ultrasonic generation at 300°C. with KMnO₄ as a precursor to MnO_(x) showed no indication of anycrystalline structures. In general, this result indicates that either nosuch species are formed or that their concentration and/or size are toosmall to be detected by XRD. Typically, for dispersed oxides, the XRDdetection limit is about a 40 to 50 nanometer crystallite size. ForMn(NO₃)₂ based catalyst (conversion temperature of 300° C.) anindication of some crystalline structures was observed. However, thefeatures were too weak for identification and, as XPS data suggested,this could be related to the presence of non-converted Mn(NO₃)₂ in thecatalysts.

A further increase in the conversion temperature produced morepronounced XRD peaks, the positions of which were related to theformation of crystalline Mn₃O₄ or Mn₂O₃. In general, this indicates thatif the conversion temperature is too high (at otherwise identicalresidence time), the diffusion and agglomeration of the convertedMnO_(x) species leads to the formation of large crystallites that areXRD detectable. Once such low-dispersion structures are formed, nosignificant electrocatalytic activity is expected. Therefore, only aproper combination of several spray generation parameters such as themethod of generation, the precursor composition and the temperature ofconversion ensures proper kinetics of the conversion and diffusion ofthe active surface species that are optimal for the electrocatalyticperformance.

The benefit of the XPS derived dispersion data relates to informationaveraged over a large number of catalyst particles. TransmissionElectron Microscopy (TEM), which gives a high magnification image of thecatalyst morphology, was used to confirm the data collected by XPS. ForExample 23A, small 1 to 2 nanometer-size amorphous MnO_(x) species wereobserved on the primary carbon particle surface (d=30 nm). EnergyDispersive Spectroscopy (EDS) performed on the particle confirmed thatthe surface species are Mn containing and that these species areuniformly distributed throughout the particle.

TEM images of Sample 2A showed areas of higher contrast attributed tothe MnO_(x) particles deposited mainly on the external surface area ofthe carbon support. Higher magnification images showed that theseMnO_(x) species are 20 to 40 nanometers in size, they are crystallineand reside on the outside surface area of the carbon support. Therefore,the TEM observations confirm the XPS derived data on the MnO_(x)dispersion and deposition uniformity. Observations by TEM for Sample 1Ayielded similar results.

TEM observations of Example 30D, which is a post heat-treated samplecorresponding to Example 30A, were also made. As described in Table X,the XPS estimated particle size for Example 30A is approximately 10 to15 nanometers, while for Example 30D it is approximately 40 nanometers.The TEM images indicated that in some areas of the carbon support, theMnO_(x) clusters are highly dispersed while in other areas largecrystallites of about 50 nanometers in size are formed. The largerclusters consist of MnO_(x) crystallites which are about 10 to 15nanometers in size.

This TEM observation is in excellent correlation with the XPS estimatedaverage particle size of 40 nanometers. In addition, these results pointto a very important detail of the structure of the electrocatalysts ofthe present invention—elevated temperature post treatment of thecatalysts may lead to diffusion of MnO_(x) crystallites, formation oflarge crystallites and significant decrease in the dispersion of theactive phase. In combination with the change in the Mn oxidation state,observed by XPS, this is a clear explanation of why the electrocatalyticactivity of a post heat-treated sample is significantly lower comparedto the original spray converted counterpart.

The influence on the electrocatalytic activity of the spray conversiontemperature, presence of surfactant, precursor concentration andadditives was systematically analyzed for Examples 41A through 47E.Based on the previous XPS findings the samples were characterized by thebinding energy positions for Mn as an indication for the Mn oxidationstate and the I (Mn)/I (C) relative intensities as a measure for thedispersion of the MnO_(x) species. The relative intensities and type ofO 1s XPS peaks were analyzed in relation to the presence of surfactantand its influence on the electrocatalytic activity.

FIG. 59 illustrates the relationship between the electrocatalyticactivity and the XPS relative intensities for Examples 19A through 30D.This dependence was used as a baseline for the further analysis of theinfluence of different spray conversion parameters on theelectrocatalytic activity. For all spray nozzle generated samples, theBET surface areas are comparable and would not significantly influencethe XPS model calculations of the average MnO_(x) cluster size.Therefore, a comparison of the XPS I (Mn)/I (C) relative intensities isan adequate measure of the dispersion of the active species. As can beseen from FIG. 59, the higher I (Mn)/I (C) relative intensities, thehigher the electrocatalytic performance of the samples, if othercharacteristics of the catalyst (surface area and Mn oxidation state)are identical.

TABLE XI XPS data for Examples 41A–47E Spray Mn 2p_(3/2) Voltage [V] atconversion Precursor binding I (Mn 2p)/ discharge temperature MnConcentration energy I (C 1s) current of Example (° C.) (wt. %) (wt. %)(eV) (Experimental) 300 mA/cm² 41A 315 10 5 642.2 0.71 0.91 41B 315 10 5642.0 0.83 0.97 41C 315 20 5 641.9 1.39 0.85 41D 315 10 5 641.8 1.030.96 44C 208  5 5 641.7 0.33 0.99 44D 149  5 5 642.3 0.46 0.92 44E 14910 5 642.3 1.03 1.01 44F 208 10 5 642.3 0.76 0.95 44G 208 10 5 642.20.86 1.00 47D 208 10 2.5 642.4 0.78 0.95 47E 208 10 2.1 642.2 0.86 0.97

The Mn 2p_(3/2) binding energy for most of the samples is identical tothe previously analyzed electrocatalysts (642.3±0.1 eV), which indicatesa Mn (IV) oxidation state. Only for Examples 41C–44C does the bindingenergy deviate significantly from the above position and is 641.8±0.1eV. Therefore, in the latter samples, the Mn oxidation state is lowerand most likely a mixture of Mn (IV) and Mn (III) oxidation states.

Examples 41A through 41D illustrate the influence of variable amounts ofsurfactant in the spray solution, variation in the weight percent of Mnas well as the spray conversion temperature as compared to Examples 30Aand 30C. A comparison between Examples 30C and 41A leads to theconclusion that a higher conversion temperature is not necessarilybeneficial for the catalyst morphology and performance. Both the MnO_(x)dispersion and the electrocatalytic activity are lower for Example 41A,which was made at a higher conversion temperature with all otherparameters being kept constant.

It appears that the lower the amount of added surfactant, the better thedispersity and electrocatalytic performance (compare Example 41A toExample 41B). Doubling the Mn concentration does not lead to anyimprovement in the electrocatalytic performance. In contrast, it is thelowest in the series of spray nozzle generated samples. This resultindicates that the surface coverage at 10 weight percent Mn and about 25m²/g support surface area has the adequate balance of well-dispersedMnO_(x) species and non-covered carbon surface area. Further increase inthe Mn concentration, even if it ensures higher concentration of MnO_(x)centers, does not improve the electrocatalytic performance.

The preparation conditions for Examples 41A and 41D are identical,except that for Example 41D a reduced flow in the spray reactor wasemployed. It appears that both the dispersion and the electrocatalyticactivity are improved for Example 41D. This effect is significant (over30%) for the dispersion and moderate for the electrocatalytic activity.It should be noted that the longer residence time leads to a loweroxidation state of Mn similar to the post heat-treated Example 30D. Thehigher residence time at elevated temperatures may lead to anundesirable reduction of the MnO_(x) surface species.

Comparison between Examples 44C and 44D, both with 5 weight percent Mnconcentration, shows that decreasing the conversion temperature from208° C. to 149° C. produced a better dispersion of the active speciesbut not necessarily better catalytic activity (Table XI). Thisobservation is confirmed for 10 weight percent Mn catalysts, Example 44E(149° C.) and Example 44G (208° C.). The two examples have significantlydifferent dispersion, the one for Example 44E being 30% higher, butidentical electrocatalytic performance. An explanation for thisdeviation of the correlation higher dispersion-higher activity can befound in the O 1s XPS spectra.

It is clear from this XPS data that the higher concentration of KMnO₄ inthe precursor solution for Example 44E had an oxidizing effect on thesurfactant present in the solution. The relative intensity of the O 1speak related to the surfactant (533.2 eV) for Example 44E is much lowercompared to Example 44D. Thus, even though the conversion temperature israther low (149° C.), the burnout of the surfactant is quite effective.Apparently, if the surfactant is still present at the catalyst surface,it blocks active centers and even though the MnO_(x) dispersion isreasonable, the activity is lower than for Example 44D. Furthercomparison with the O 1s XPS spectra for Example 47D shows that if theadded surfactant is completely eliminated, the O 1s peak related to thesurfactant is further decreased in intensity.

Lowering the surfactant concentration in the precursor solution has anegative effect on the MnO_(x) cluster dispersion (compare Example 30Cand Example 47D). However, the benefit of less surfactant, which ifpresent acts as a catalyst poison, outweighs the lower dispersion andthe resulting catalytic activity is identical. Therefore, the amount ofhigh molecular weight surfactants should be minimized in the precursorsuspensions.

Example 44E prepared with spray nozzle generation has identically highperformance to the ultrasonically generated Example 23A. Example 44E wasprepared at the lowest conversion temperature of 149° C., with minimaladdition of surfactant, 10 weight percent Mn and 5 weight percentsolution concentration. Most likely these particular conditions (alongwith others such as residence time) ensure good kinetic conditions forthe formation and distribution of the MnO_(x) active species on thecarbon support.

A higher surface area carbon support compared to the carbon support usedfor the previous spray conversion examples (surface area of 254 m²/g)will provide higher support surface area available for the MnO_(x)absorption. The surface area reduction after the spray conversionobserved for the previous carbon support was on the order of about 10times. Because of that reduction of the surface area, higher loading ofMnO_(x) was not beneficial for the electrocatalytic activity because ofthe lack of sufficient surface area to ensure high dispersion of theadditional amounts of MnO_(x). Therefore, carbon supports with higherstarting surface area and different types of porosity were chosen totest their applicability for producing electrocatalysts with higherMnO_(x) loading and high dispersion of the active MnO_(x) species.

Electrocatalyst preparation was again conducted using ultrasonic andtwo-fluid jet nozzle aerosol generation and two types of high surfacearea carbons were used, KET JENBLACK (Akzo Nobel, Ltd, Amersfoort,Netherlands) and BLACKPEARLS 2000 (Cabot Corp., Alpharetta, Ga.). Eachhas a surface area of from about 1300 to 1500 m²/g although the KETJENBLACK carbon is more graphitic. KET JENBLACK samples wereconsiderably more viscous and thus were diluted with water to reduce thecarbon concentration to 2 weight percent. BLACKPEARLS samples were lessviscous and were diluted with water to yield 4 weight percent carbon. Inthe samples with Mn/C ratios higher than 10%, KMnO₄ was added, beinginitially dissolved in the water used for sample dilution. All thesamples were processed while varying the inlet temperature and KMnO₄content. This resulted in the electrocatalyst samples listed in TableXII.

TABLE XII High surface area carbon electrocatalyst examples Carbon BlackMn/C Inlet T Weight Recovery Example Source (wt. %) (° C.) (g) (%) 35AKetjenblack 10 208 400 82 35B Ketjenblack 10 315 440 88 36A Ketjenblack15 208 530 94 36B Ketjenblack 15 315 510 92 37A Ketjenblack 20 208 60093 37B Ketjenblack 20 315 660 94 38A Blackpearls 10 208 490 98 38BBlackpearls 10 315 500 99 39A Blackpearls 15 208 570 97 39B Blackpearls15 315 570 98 40A Blackpearls 20 208 610 96 40B Blackpearls 20 315 63098

Examples were processed in the order listed in Table XII. The carriergas pressure was 80 psi and the carbon suspension was delivered to thespray nozzle at a rate of 150 to 250 mL/min.

MnO_(x) was successfully dispersed over both high-surface area carbonmaterials resulting in high surface area electrocatalytic powder. FIGS.60 and 61 illustrate the dependencies of the surface area on the amountof Mn deposited on the carbon. Catalysts based on both types of carbonsupport can be obtained with BET surface areas from 850 to 600 m²/g whenthe Mn concentration is varied from 10 to 20 weight percent. Theconversion temperature does not affect the surface area significantly.In all cases there is an apparent linearity in the dependencies that canbe interpreted in terms of even distribution of the MnO_(x) on thecarbon carrier surface, resulting in additive blocking and agglomerationeffects.

Table XIII contains the XPS data for the examples listed in Table XII.

TABLE XIII XPS data for samples based on high-surface area carbon blacksSpray drier inlet Mn 2p_(3/2) I (Mn 2p)/ temperature Mn binding energy I(C 1s) Example (° C.) (wt. %) (eV) Experimental 35A 208 10 l. r.* 0.05235B 315 10 641.6 0.143 36A 208 15 642.8 0.229 36B 315 15 644.1 0.257 37A208 20 l. r.* 0.061 37B 315 20 643.2 0.330 38A 208 10 644.5 0.052 38 31510 l. r. 0.000 39A 208 15 643.1 0.269 39B 315 15 642.6 0.141 40A 208 20n.a. n.a. 4GB 315 20 642.2 0.244 *l. r. - low resolution

FIGS. 62 and 63 compare the dispersion of the MnO_(x) species (followedby the changes in the XPS I (Mn)/I (C) relative intensities) as afunction of the Mn loading and spray drier inlet temperature for KETJENBLACK (FIG. 62) and BLACKPEARLS (FIG. 63) supports. Since the samplesurface area also changes as a function of the active species loadingand the conversion inlet temperature, an accurate comparison of thedispersion requires application of the Kerkhof and Moulijn XPS model.However, for initial evaluation of the catalysts morphology and activephase distribution, the XPS I (Mn)/I (C) relative intensities will beused.

For both catalyst supports an inlet temperature of 315° C. producessamples with a linear increase of the XPS I (Mn)/I (C) relativeintensities vs. Mn content. For an inlet temperature of 208° C., theresults are very different for the two types of samples. While for theKET JENBLACK support the XPS values for an inlet temperature of 208° C.are lower than for 315° C. whereas, for the BLACKPEARLS support they aresignificantly higher. These results indicate that carbon supportchemistry and morphology play a significant role in the formation of theactive species and their distribution. In support of this statement arethe XPS data for the Mn 2p_(3/2) binding energy positions (Table XII).Only for very few samples such as Examples 40B, 39B, and 36A is thebinding energy similar to that of Mn (IV) oxidation state. For Example35B it is closer to the Mn (III) oxidation state, while for all othersamples it has significantly higher values. These higher values could bedue either to the presence of non-converted precursor or the presence ofconverted MnO_(x) species with higher than Mn (IV) oxidation state.

2. NiCoO_(x) Bi-functional Electrocatalysts

Bi-functional catalysts for oxygen reduction/evolution are complexelectrochemical catalyst systems. These electrocatalysts must possess atleast two different types of catalytic active centers, based on the factthat oxygen evolution and oxygen electro-reduction are both irreversiblereactions. Among several possible chemistries, the mixed oxide systemNiO:CoO (1:2) was selected for evaluation. This is one of the leastsophisticated bi-functional electrocatalyst, yet demonstratesexceptional activity and sufficient cycle life.

The cycle life of a bi-functional catalyst will be limited by thedestruction of the carbon support during oxygen evolution (cellcharging), the so called “electrochemical burning” of carbon. Graphiticcarbons are more resistant to the electrochemical oxidation duringoxygen evolution than amorphous carbons. NiO:CoO compositeelectrocatalysts are typically obtained by a conventional precipitationon activated carbon. A catalyst made by this method was used forcomparison to the present invention.

Several samples of NiO:CoO electrocatalysts were synthesized bothsupported on various carbon materials and as individual compositepowders (no supporting material used). The catalysts samples wereprepared using ultrasonic aerosol generation and the resultingelectrocatalyst powders exhibited surface areas of from about 2 to 90m²/g.

Electrochemical evaluation of the NiO:CoO bi-functional electrocatalystsincluded initial polarization curves in alkaline electrolyte forscreening and incorporation into a laboratory test cell with a metalhydride (MH) anode to form a MH-air system and to test cycle life.

The bi-functional oxygen electrodes were made using a conventionaldry-powder press technology. A hydrophobic layer of hydrophobizedacetylene carbon black (35 TEFLON) was pressed at 300 kg/cm² at anelevated temperature of 340° C. on a nickel mesh current collector witha catalytic layer, loaded with approximately 1 mg/cm² ofelectrocatalyst. Electrodes with three types of catalyst were compared:dispersed NiO:CoO electrocatalyst; NiO:CoO electrocatalysts supported oncarbon black; and a “standard” NiO:CoO electrocatalyst, made by atraditional precipitation procedure. All the electrodes were evaluatedin a half-cell testing assembly using KOH as electrolyte.

FIG. 64 illustrates the polarization curves of the electrodes tested.The polarization curve of the electrode with a prior art catalyst isshown in FIGS. 64–66 by a dashed line and the remaining lines arecatalysts produced according to the present invention. It can be seenthat dispersed NiO:CoO electrocatalyst prepared according to the presentinvention demonstrates the most advantageous performance in oxygenreduction. The superiority of the same electrocatalyst is even morepronounced in the reaction of oxygen evolution, as illustrated in FIG.65. In this case, the NiO:CoO electrocatalyst of the present inventionallows evolution of the molecular oxygen at the lowest anodic potential.

Re-calculation of the results presented in FIG. 64 and FIG. 65, in termsof ratios of the cathodic/anodic voltages at a given current density(expressed in percent) is illustrated in FIG. 66. This figure presentsthe voltaic efficiency of the oxygen bi-functional electrodes which isdirectly associated with energy losses during the charge/dischargecycles of a cell.

Due to the superior performance of the NiO:CoO electrocatalyst of thepresent invention voltaic efficiency of the electrode made with thiscatalyst is from 62 to 65 percent within the expected range of operatingcurrent densities of 10 to 20 mA/cm². This is a very promising resultsince voltaic efficiencies above 55 percent are considered practical,and in commercial battery systems they usually do not exceed 60percent;. Based on these results, the dispersed unsupported NiO:CoOelectrocatalyst was selected for further evaluation in a laboratoryMH-air cell.

Two identical bi-functional oxygen gas diffusion electrodes (open area 5cm²) with NiO:CoO electrocatalyst were used in a symmetrical assembly.They were mounted in the walls of the testing cell.

The metal hydride electrode was prepared using the AB₅ metal alloycomposition MmNi_(4.1)Co_(0.4)Mn_(0.4)Al_(0.3) as the active species(where Mm is mischmetal). The electrode consisted of 0.1 g/cm² a mixtureof the metal hydride alloy powder and hydrophobized carbon black (35 wt.% TEFLON) in a weight ratio of 1:1, pressed on the both sides of anickel mesh. The working area of the electrodes was 5 cm².

The MH electrode was immersed between the two oxygen electrodes and thecell was filled in with a 31 wt. % KOH electrolyte. The overallthickness of the test cell is approximately 11 mm (1 mm per electrodeand 4 mm for each electrolyte layer, the distance between the oxygencathode and the MH anode). The central part of the cell was providedwith two holes for filling the electrolyte and for a reference electrodeused for independent measurements of the electrodes polarization duringoperation. The electrolyte volume in the cell was 10 cm³.

The MH-air testing cell was cycled at relatively high current densitiesof 13.5 mA/cm² at C/2 charge/discharge rate (total current approximately110 mA). This regime was chosen to expose the oxygen electrodes to harshconditions in order to reveal a potential malfunction in a considerablyshort time. FIG. 67 shows the voltage transients obtained during severaldays of such a high-load test. It can be seen from the figure that thecell performance did not deviate from uniformity through 16charge/discharge cycles. There was no observation of the electrolytedarkening from eventual products of oxygen electrode degradation. Theelectrocatalyst has demonstrated superior electrochemical activity withrespect to both reactions and contains no oxidizable carbon support.

Reproducibility and the effectiveness of the charge/dischargecharacteristics of the testing MH-air cell are illustrated also by FIG.68, which shows the specific capacity as a function of 24 cycles. Thecurve reveals excellent initial cyclabilty of the cell demonstrated bythe retention of the discharge capacity at 220 mAh/g (variations areassociated with periodic additions of electrolyte). There is nodeclining trend after 15 to 20 cycles, which is frequently observed forother metal-air secondary battery systems. The gravimetric power densityis about 35 W/kg and the volumetric power density is 70 W/L. Thecorresponding energy density is 80 Wh/kg and specific energy is 150Wh/L.

3. Metal-Carbon Composite Powders

Further examples in accordance with the present invention were preparedand are described in Table XIV. The powder batch examples were preparedby ultrasonic generation and the aerosol was heated in a tubular furnacereactor. All of the examples were prepared using GRAFO 1300 (FuchsLubricant Co., Harvey, Ill.) which is a suspension of carbon particleshaving an average particle size of 30 nanometers and a surface area ofabout 254 m²/g. The corresponding amount of Pt precursor was dissolvedand added to the carbon suspension to form the precursor. Table XIVdescribes the type of Pt precursor used, the carrier gas, the conversiontemperature and targeted Pt nominal concentration in the final catalyst.

TABLE XIV Conditions for ultrasonically generated Pt/C powder Furnace Pttemperature Example Pt precursor (wt. %) (° C.) Carrier gas 27BPt(NH₃)₄(NO₃)₂ 20 400 Air 31B Pt(NH₃)₄(NO₃)₂ 20 700 Air 31CPt(NH₃)₄(NO₃)₂ 20 500 Air 32A Pt(NH₃)₄(NO₃)₂ 20 300 Air 32BPt(NH₃)₄(NO₃)₂ 20 200 Air 33B Pt(NH₃)₄(NO₃)₂ 20 200 N₂ 33CPt(NH₃)₄(NO₃)₂ 20 300 N₂ 36A Pt(NH₃)₄(NO₃)₂ 20 300 N₂ 36B Pt(NH₃)₄(NO₃)₂20 300 N₂ 36C Pt(NH₃)₄(NO₃)₂ 20 500 N₂ 37A Pt(NH₃)₄(NO₃)₂ 20 500 N₂ 37BPt(NH₃)₄(NO₃)₂ 20 500 N₂ 37C Pt(NH₃)₄(NO₃)₂ 20 700 N₂ 37D Pt(NH₃)₄(NO₃)₂20 700 N₂ 37E Pt(NH₃)₄(NO₃)₂ 20 700 N₂ 38A Pt(NH₃)₄(NO₃)₂ 20 500 N₂ 38BPt(NH₃)₄(NO₃)₂ 20 500 Air 39A Pt(NH₃)₄(NO₃)₂ 20 400 Air 39B H₂Pt(OH)₆ 10400 Air 40C H₂Pt(OH)₆ 10 300 Air

FIG. 69 illustrates a low magnification (2000×) TEM image of Example31C, which is typical for the examples produced. The secondary carbonparticles are substantially spherical with the particle size varyingbetween 1 and 2 μm. The secondary particles (support phase) consist ofprimary carbon particles of about 30 nanometer diameter and varioussizes of Pt particles and particle clusters dispersed thereon. FIG. 70illustrates a higher magnification image (12,000×) of Example 31C. Ascan be seen from the TEM images, the secondary electrocatalyst particleshave a highly porous structure.

The BET nitrogen absorption method was used to analyze the surface areaof the ultrasonically generated Pt/C catalyst powders according to thepresent invention. The results are summarized in FIG. 71. Both theconversion temperature and the carrier gas composition had an effect onthe catalyst surface area. When air is used as a carrier gas, thesurface area is higher at a conversion temperature of 300° C. (89 m²/g)compared to 200° C. (22 m²/g). However, a further increase of theconversion temperature to 400° C. did not lead to significant change inthe surface area. In contrast, when nitrogen is used as carrier gas, thecatalyst surface area increases to 125 m²/g at 500° C. and a furtherincrease of the conversion temperature to 700° C. also decreases thesurface area.

Analysis of the changes in the surface area as a function of the sprayconversion temperature and carrier gas composition led to the followingconclusions:

-   -   when air is used as a carrier gas, spray conversion temperatures        above 300° C. are not significantly beneficial for increasing        the surface area;    -   when nitrogen is used as a carrier gas, the powder surface area        is generally higher compared to powders generated with air as a        carrier gas;    -   if nitrogen is used as a carrier gas, a conversion temperature        of 500° C. is advantageous for producing a high surface area        powder; and    -   the surface area after spray conversion is at least three times        lower than the surface area of the original carbon support.

XPS analysis was performed on the samples to provide information aboutthe Pt oxidation state and dispersion in the catalysts. Three maincharacteristics of the XPS spectra were analyzed allowing comparisonbetween the samples generated at different conditions: the positions ofthe binding energy of Pt 4f_(7/2) photoelectrons which indicates the Ptoxidation state; the relative intensities of Pt 4f vs. C 1sphotoelectron peaks which indicates the level of Pt dispersion; and theappearance of N 1s photoelectron peak and its relative intensity vs. C1s peak which indicate the level of impurities and the degree ofconversion of the platinum precursor.

All preparation conditions, recording of the spectra and data processingwere identical for all samples. The samples were prepared for XPSanalysis by pressing them into indium foil (99.9%) which was previouslycleaned in HNO₃ to remove impurities at the surface.

The XPS spectra for all of the catalysts were recorded on an AXIS HSi(Kratos Analytical) spectrometer, working in ΔE=constant mode at a passenergy of 80 eV, using a monochromated aluminum anode (Al K_(α)=1486.7eV, 225 W). The residual pressure in the analysis chamber was 1×10⁻⁹Torr (1 Torr=133.3 Pa). The peak positions were estimated relative tothe binding energy of C 1s=284.6 eV. The following XPS peaks, designatedby their electron levels, were recorded: Pt 4f, C 1s, O 1s and N is. Onesurvey scan was acquired in the 75 to 1175 eV binding energy rangebefore the high resolution spectra were acquired. The experimentalintensities were estimated from the areas of the corresponding peaks,measured on smoothed original peaks. The area of the Pt 4f peak includesthe areas of both Pt 4f_(7/2) and 4f_(5/2) peaks. The results are listedin Table XV.

As listed in Table XV, a Pt 4f_(7/2) binding energy of 71.2 eV wasmeasured for the prior art catalyst (Sample 3A). The measured values forthe Pt 4f binding energies, peak hwhm (half width at half maximum) andpeak intensities closely match the theoretical and model XPS handbookvalues of Pt 4f peaks for Pt (0) oxidation state, i.e., for Pt metal.These values were further used for curve fitting of the Pt 4f peaks forthe catalysts according to the present invention.

TABLE XV XPS data for Pt/C Powders Pt 4f_(7/2) peak position I (Pt 4f)/I(C 1s) I (N 1s)/I (C 1s) Example (eV) (relative intensities) (relativeintensities) Sample 3A* 71.2 0.682 0.028 (1.5 at. %) 27B 71.2 (80%)0.305 0.000 72.5 (20%) 31C 71.4 0.481 0.020 (1.0 at. %) 32A 72.1 (80%)0.398 0.000 73.2 (20%) 32B 73.4 0.352 0.149 (5.6 at. %) 33B 73.3 0.4060.165 (7.0 at. %) 37C 71.8 0.489 0.009 (0.4 at. %) 38A 71.6 0.525 0.00039A 71.7 0.327 0.000 39B 71.6 0.234 0.022 (1.1 at. %) 40C 71.9 0.3270.025 (1.3 at. %) *prior art

Table XV also contains information regarding the I (Pt 4f)/I (C 1s)relative intensities, which can be used to measure the dispersion of thePt clusters on the carbon support. Since almost all of the catalysts ofthe examples contain an identical amount of Pt as Example 3A (20 wt. %Pt), the relative intensities I (Pt 4f)/I (C 1s) can be used for directcomparison of their Pt dispersion relative to the one for the commercialcatalyst. This is mostly accurate for the catalysts of the presentinvention that have comparable surface area to the commercial catalyst,e.g., those prepared at spray conversion temperatures of 300° C. andabove in nitrogen and at 400° C. and above in air.

Table XV also lists the relative intensities I (N 1s)/I (C 1s) and thesurface concentration of nitrogen in atomic percent for all catalystsanalyzed by XPS. Example 3A (prior art) contains small impurities ofnitrogen, which could indicate the use of nitrogen-containing reagentsin the preparation of the catalyst or the presence of anitrogen-containing surfactant.

As can be seen from the Pt 4f spectrum for Example 27B, the curve fitfor the Pt 4f peaks cannot be accomplished using only the doubletrelated to Pt(0) oxidation state. A second doublet of Pt 4f peaks isneeded with binding energy of 72.5 eV for the Pt 4f_(7/2) peak. This Pt4f_(7/2) binding energy can be related to Pt (II) oxidation stateindicating that the conversion of the Pt precursor to Pt metal is notcomplete in Example 27B. The relative intensity of the second doubletrelated to Pt (II) oxidation state accounts for approximately 20% of thetotal Pt 4f peak area and therefore up to 20% of the Pt in the Example27B is not converted to Pt (0) oxidation state, indicating that sprayconversion in air at 400° C. does not completely reduce the Pt precursorto Pt(0) and does not produce good dispersion of Pt clusters on thecarbon support. The value of the relative intensity I (Pt 4f)/I (C 1s)for Example 27B is more than two times lower compared to the one for thecommercial catalyst. No nitrogen impurities, however, were detected forExample 27B.

It should be noted that for Pt-based fuel cell catalysts, supported oncarbon, highly dispersed Pt metal clusters are required for achievinghigh catalytic activity. Therefore, achieving high dispersion of Pt inthe Pt (0) state can be used as criteria for the prediction of catalyticperformance of the fuel cell catalysts.

In order to find optimal spray conversion conditions for achievingcomplete Pt reduction and high dispersion, the changes in thesecharacteristics as a function of the spray conversion temperature andthe carrier gas composition were analyzed. In general, a shift in theposition of the Pt 4f_(7/2) peak towards higher than 71.2 eV bindingenergies was considered an indication of a non-complete reduction to Ptmetal. Simultaneously, a relative intensity I (Pt 4f)/I (C 1s) lowerthan the commercial sample is indicative of lower Pt cluster dispersion,corresponding to higher average size of Pt clusters. For Example 38A,the XPS analysis was repeated in order to estimate the accuracy of themeasurements. A comparison between the two analyses shows excellentreproducibility for the XPS peak positions and less than 2% differencein the XPS relative intensities.

FIG. 72 illustrates the dependence of the Pt 4f_(7/2) binding energyposition for the catalysts (formed from Pt(NH₃)₄(NO₃)₂ precursor) as afunction of the spray conversion temperature and the carrier gascomposition. A conversion temperature of at least 500° C. is necessarywith air as a carrier gas to achieve a reasonably high degree ofconversion to the Pt (0) oxidation state. There are no significantdifferences observed when nitrogen is used as a carrier gas. An increaseof the conversion temperature to 700° C. does not lead to improvedresults. Therefore, when Pt(NH₃)₄(NO₃)₂precursor is used in theformulations, a temperature of at least about 500° C. seems adequate forachieving complete conversion of the precursor and formation of Pt metalspecies.

In support of that conclusion are the high-resolution XPS spectra whichwere measured for Example 32B and Example 33B. Both catalysts were madeat a conversion temperature of 200° C., Example 32B with air as acarrier gas and Example 33B with nitrogen as a carrier gas. In additionto the peaks at 73.2 eV for Pt(II) oxidation state, e.g., partiallyconverted precursor, another Pt 4f peak doublet appears with Pt 4f_(7/2)binding energy of approximately 75.6 eV which is very close to the valuefor the Pt (IV) oxidation state. In the N 1s region peaks at 404.2 eVand 406.5 eV are observed and related to NO₂ and NO₃ species, whichconfirms the conclusion for non-complete precursor conversion. Thisnon-complete conversion results in a concentration of nitrogen in thesecatalysts of up to 7 atomic percent.

An increase in the conversion temperature to 300° C. (Example 32A) leadsto a significant decrease of the nitrogen impurities. However, eventhough the precursor conversion is more complete, approximately 20% ofthe Pt in the Pt(II) oxidation state.

As mentioned above, the dispersion of the Pt clusters is of significantimportance for achieving high catalytic activity. FIG. 73 illustratesthe dependence of the relative intensities I (Pt 4f)/I (C 1s) as afunction of spray conversion temperature. In general, at identical sprayconversion temperatures, the catalysts made with nitrogen as a carriergas show higher relative intensities I (Pt 4f)/I (C 1s) and thereforehave better dispersion of Pt on the support surface. An increase of thespray conversion temperature up to 500° C. leads to improved Ptdispersion for both air and nitrogen as the carrier gases. Increasingthe spray conversion temperature to 700° C. is not beneficial for the Ptdispersion. The highest I (Pt 4f)/I (C 1s) relative intensity value isobserved for Example 38A, which was prepared at 500° C. conversiontemperature in nitrogen. The relative intensity I (Pt 4f)/I (C 1s) of0.525 for Example 38A is still lower than the one measured for Sample3A, which suggests lower dispersion. However, no nitrogen impurities aredetected for Example 38A, while about 1.5 atomic percent impurities weredetected for Sample 3A.

Two of the samples listed in Table XV were synthesized with a differentPt precursor (H₂Pt(OH)₆), with only 10 weight percent Pt. The XPS datafor Examples 39B and 40C illustrate that a reaction temperature of atleast 400° C. in air is necessary for achieving the Pt (0) oxidationstate from this precursor. The Pt has higher dispersion for Example 40C,which was prepared at 300° C. compared to Example 39B, prepared at 400°C. This result is in contrast with the XPS data for samples based onPt(NH₃)₄(NO₃)₂ precursor, for which higher conversion temperatures ledto better Pt dispersion. This result suggests that H₂Pt(OH)₆ precursorconverts at lower temperatures compared to Pt(NH₃)₄(NO₃)₂, andundesirable diffusion and agglomeration of Pt clusters occurs at higherconversion temperatures.

XPS analysis of the electrocatalyst powders provides informationregarding important characteristics of the catalysts such as Ptoxidation state and dispersion, which influence the catalytic activityof the powders. However, other characteristics of the catalysts such asPt cluster size distribution may have significant impact on thecatalytic activity as well. Since XPS data gives only an estimate of theaverage metal dispersion, other methods such as TEM and hydrogenchemisorption can be employed for further information about the Pt/Ccatalyst structure.

FIGS. 74 and 75 illustrate high magnification TEM images for Example 3A(prior art). The catalyst has good overall uniformity of thedistribution of Pt clusters on the carbon support particles. The Ptcluster size distribution is very narrow and most of Pt clusters have adiameter between about 3 and 5 nanometers.

In contrast, the overall cluster size distribution for the Pt/Celectrocatalyst powders of the present invention is not as uniform.FIGS. 76 and 77 illustrate high resolution TEM images for Example 31C(FIG. 76) and Example 38A (FIG. 77). For both samples, agglomeration ofthe Pt clusters is observed in some area of the catalysts, while lowcoverage with Pt clusters is observed in other areas. This apparentlyleads to the lower overall dispersion of Pt as found by XPS analysis.

However, the overall distribution for Example 38A (FIG. 77), which wasspray converted at 500° C. in nitrogen, is significantly better comparedto Example 31 C (FIG. 76), which was spray converted at 500° C. in air.This observation is in agreement with the XPS data for Pt dispersion andconfirms that carrier gas has influence on the catalyst formation and Ptdispersion in particular.

FIGS. 78 and 79 illustrate that Pt cluster size distribution forcatalysts of the present invention is much broader than for Sample 3A,with Pt cluster diameter varying between 1 and 10 nanometers. However, asignificant number of Pt clusters with a size below 3 nanometers areobserved. Comparison between FIG. 74 (Sample 3A) and FIG. 78 (Example39A) illustrates that for Example 39A, Pt clusters of about 1 to 2nanometers are observed.

FIGS. 80 and 81 illustrate high magnification images of Example 39B,synthesized with H₂Pt(OH)₆ precursor. Comparing FIG. 78 and FIG. 80,which were prepared with two different precursors at identicalconversion temperatures the degree of non-uniform deposition issignificant for Example 39B. When H₂Pt(OH)₆ precursor was used, thedegree of agglomeration of Pt clusters is much higher, likely due toenhanced diffusion of the Pt clusters. Since the decompositiontemperature for the H₂Pt(OH)₆ precursor is lower, the Pt clusters areformed earlier in the spray conversion process and have a longer time todiffuse and form agglomerates.

Samples of the Pt/C electrocatalysts were evaluated in PEM fuel cellsand the results of the electrochemical characterization were compared totwo commercially available electrocatalysts. The Examples which wereelectrochemically characterized are Examples 37C, 38A, 39A and 39B.

Gas diffusion cathodes were fabricated by the catalyst ink method. ThePt/C catalyst was dispersed in a Nafion/alcohol/water solution to give astable ink suspension. Specifically, 1 g of the electrocatalyst wasmixed in 2 ml i-propanol (after being wetted with a small amount ofwater to avoid pyrogenic effects), and suspended in 10 ml of stockNafion solution (5 wt. % of polymer in water/i-propanol mix). This inkyields a Catalyst/Nafion ratio of 2:1, which is to remain during theelectrode preparation in order to incorporate the electrocatalystparticles into the Nafion polymer electrolyte membrane.

The gas diffusion electrode is prepared by brush application of asuspension of non-porous acetylene carbon black (Shawinigan Black,Chevron Chem. Co., Houston, Tex.), and TEFLON emulsion (DuPont) to givea 35 to 40 weight percent TEFLON/carbon ratio onto a carbon cloth. Thegas-diffusion electrode, soaked with the TEFLON/carbon suspension, isheat treated at 300° C. to 350° C. for 1 hour. This temperature range isnear the glass-transition point of the TEFLON material.

The Pt/Carbon electrocatalyst ink is applied on the impregnated cloth bya brush when the electrode is mounted on a hot plate at 90 to 100° C.The electrode is then treated at 155° C. in air for 20 to 30 minutes,which is close to the melting point of Nafion material. The catalystloading is determined from the electrode weight.

The platinum loading of the cathodes was 0.20±0.01 mg/cm²which isconsidered tow by industrial standards for oxygen electrocatalysts. Allhydrogen electrodes (anodes) were loaded with 0.05 mg/cm² of platinumusing a 10% Pt/C commercial catalyst.

Membrane electrodes assemblies (MEAs) were fabricated by hot pressingelectrodes symmetrically (catalyst side facing the membrane) onto bothsides of a Nafion 112(CG Processing, Inc.) PEM at 200° C., to allowmelting of the membrane and the Nafion material from the catalyticlayers. The performance evaluation of MEAs was carried out in test cellwith a working area of 50 cm² between ribbed graphite plates and copperend plates at 50° C. and an atmospheric pressure of humidified reactantgases.

FIG. 82 illustrates a comparison of voltamograms (cell potential vs.current density plots) for MEAs comprising different commercialcatalysts (Samples 4A and 5A) and a catalyst according to the presentinvention, prepared and measured under identical conditions. The resultswere obtained with electrocatalysts containing 20 weight percentplatinum on an identical carbon black support. It is evident from thesecurves that the electrocatalyst of the present invention demonstratessuperior performance in the MEA. Numerical expression of thissuperiority can be derived from the current density corresponding to acell potential of 0.6 V. Both prior art electrocatalysts provide about400 mA/cm² while the electrocatalyst of the present invention provides600 mA/cm², a 50% improvement of MEA performance at a cell potential of0.6 V.

FIG. 82 illustrates that the electrode fabricated with theelectrocatalyst of the present invention demonstrates overall highercurrent densities within the entire investigated range of potentials. Atthe same time, the polarization curve is characterized by lowerdependence of the current on the potential (lower negative slope of thecurve in its “linear” portion), which indicates lower ohmic resistanceof the catalytic layer. The dependence of the potential on currentdensity remains practically linear even at high current densities,indicating that there is no expression of any diffusion limitations inthe investigated current density range.

FIG. 83 is a Tafel plot of the data from the low current density regionof FIG. 82. A Tafel plot is a semi-logarithmic data representation usedto establish the mechanism of the reaction (from the negative slope ofthe linearized dependencies) and to reveal the catalytic effectsexpressed as the position of the intercepts on the current density axis.The off-gas from the reactor, or a portion thereof, can advantageouslybe recycled to conserve gas quantities. The recycled gas can be treatedto remove unwanted components and fresh H₂. FIG. 83 illustrates that theadvantageous performance of the catalyst generally revealed in FIG. 82is due to higher catalytic activity. All three curves are linear (insemi-logarithmic coordinates) with the same negative slope, suggesting auniform mechanism of oxygen reduction. The curve corresponding to theelectrocatalyst of the present invention, however, is shifted towardhigher current densities with a positive difference in the currentdensity axis cutoff of approximately 30 mA/cm². Both prior art samplesdemonstrate practically identical catalytic activity.

The improvement in catalytic activity of the electrocatalyst of thepresent invention when compared to the prior art samples can beexplained by the platinum cluster size and its distribution on thecarbon surface. SEM microphotographs of the electrocatalysts of thepresent invention compared to the prior art electrocatalyst show thatthe catalyst of the present invention possesses a significant amount ofsmaller size platinum clusters (1–2 nm) compared to the prior artsamples. This results in an increased platinum utilization and a largerreaction interface in the active layer of the oxygen electrode.

FIG. 84 is a comparison of the polarization curves obtained with a Pt/Ccatalyst of the present invention (20 weight percent Pt) with the bestperforming research sample known to the present inventors, a Pt—Co—Cr/Ccomposite electrocatalyst (Sample 6A). This catalyst is applied to theelectrode surface to give identical loading, measured as Pt metal percm². Due to the amount of Pt in the electrocatalyst of the presentinvention, the electrode is loaded with 3-times less catalyst than theSample 6A. The polarization curves of both electrodes practicallycoincide demonstrating unique matching of the performance of an advancedtri-metal composite catalyst by the simpler Pt electrocatalyst of thepresent invention.

FIG. 85 compares the polarization curve obtained with another prior artelectrocatalyst (Sample 7A) with an example of the present inventionwith the same Pt content. The curves are obtained with different Ptloadings of 0.21 mg/cm² for Sample 7A and 0.11 mg/cm² for theelectrocatalyst of the present invention. Coinciding curves are observedwhen the Pt loading of the electrocatalyst of the present invention isabout half of the amount of the commercial catalyst. This clearlyindicates a significant commercial advantage of the present invention:the Pt/Carbon catalyst meets the same performance achieved with half ofthe amount of the catalyst material, thus significantly reducing theamount of Pt used with no loss in performance.

FIG. 86 illustrates the performance of two examples of the presentinvention with different Pt content when ambient air is used to feed theoxygen gas diffusion electrode in the cell. As expected, theelectrocatalyst with the lower Pt content (10% Pt/Carbon) provides lowercurrent densities compared to the one with higher metal content (20%Pt/Carbon). It should be noted, however, that the curves are obtainedwith electrodes that have been prepared with identical total amount ofPt. Thus, the lower Pt content sample has been applied in an amountdoubling the use of the catalyst. Reduction of the electrochemicalperformances however, is still to the level of those obtained with theprior art electrocatalysts (compare FIG. 86 and FIG. 82). The 10%Pt/Carbon sample curve of the present invention overlaps with the 20%Pt/Carbon prior art samples.

FIG. 87 is obtained with the same MEA as FIG. 87 and illustrates theperformance of the electrocatalysts of the present invention withdifferent Pt content when pure oxygen is used to feed the oxygen gasdiffusion electrode in the cell. Flowing pure oxygen through theelectrode largely eliminates the mass transport limitations, especiallythose associated with macro-diffusion processes. The curve obtained fromthe electrocatalysts with lower Pt content (10% Pt/Carbon) is shifted toapproximate the one obtained from the catalyst with higher metal content(20% Pt/Carbon). Thus, FIG. 87 demonstrates that lower performance ofthe 10% sample (as illustrated in FIG. 86) is associated with thethickness of the catalytic layer formed when double the amount ofmaterial is used. This is confirmed by the Tafel plot of the data at lowcurrent densities (where the catalytic performance is not masked by thetransport processes) as illustrated in FIG. 88.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. However, it is to beexpressly understood that such modifications and adaptations are withinthe spirit and scope of the present invention.

1. A metal-air battery comprising an anode and a cathode, wherein saidcathode comprises: a) a gas diffusion layer; b) a current collector; andc) a gradient functional layer, wherein said gradient functional layercomprises polymer-modified carbon particles having an average size ofnot greater than about 10 μm and electrocatalyst particles and whereinthe concentration of said polymer-modified carbon particles increaseswith increased distance from said anode and wherein said currentcollector and said functional gradient layer have a total averagethickness of not greater than about 50 μm.
 2. A metal-air battery asrecited in claim 1, wherein said gas diffusion layer comprises a poroustetrafluoroethylene fluorocarbon polymer.
 3. A metal-air battery asrecited in claim 1, wherein said current collector comprises elongatestrips of a metal.
 4. A metal-air battery as recited in claim 1, whereinsaid current collector comprises elongated strips of a metal having anaverage width of not greater than about 100 μm.
 5. A metal-air batteryas recited in claim 1, wherein said gradient functional layer comprisesa tetrafluoroethylene fluorocarbon polymer.
 6. A metal-air battery asrecited in claim 1, wherein said functional gradient layer comprises ahydrophobicity gradient.
 7. A metal-air battery as recited in claim 1,wherein said electrocatalyst particles comprise carbon compositeparticles.
 8. A metal-air battery as recited in claim 1, wherein saidelectrocatalyst particles have an average particle size of not greaterthan about 10 μm.
 9. A metal-air battery comprising an anode and acathode for the reduction of oxygen, wherein said cathode comprises: a)a gas diffusion layer; b) a current collector; and c) an electrocatalystlayer in electrical contact with said current collector, wherein saidelectrocatalyst layer comprises carbon particles and electrocatalystparticles dispersed throughout a polymer matrix wherein thehydrophobicity of said polymer matrix increases with increased distancefrom said anode; wherein said current collector and said electrocatalystlayer have a combined average thickness of not greater than about 100μm.
 10. A metal-air battery as recited in claim 9, wherein said gasdiffusion layer comprises a porous tetrafluoroethylene fluorocarbonpolymer.
 11. A metal-air battery as recited in claim 9, wherein saidcurrent collector is a metal mesh.
 12. A metal-air battery as recited inclaim 9, wherein said current collector comprises elongate strips of ametal.
 13. A metal-air battery as recited in claim 9, wherein saidcurrent collector comprises elongate strips of a metal and wherein saidstrips have an average width of not greater than about 100 μm.
 14. Ametal-air battery as recited in claim 9, wherein said carbon particleshave an average particle size of not greater than about 10 μm.
 15. Ametal-air battery as recited in claim 9, wherein said polymer matrixcomprises a tetrafluoroethylene fluorocarbon polymer.
 16. A metal-airbattery as recited in claim 9, wherein said polymer matrix comprises atetrafluoroethylene fluorocarbon polymer and wherein the amount of saidtetrafluoroethylene polymer in said polymer matrix increases withincreased distance from said anode.
 17. A metal-air battery as recitedin claim 9, wherein said electrocatalyst particles comprise carboncomposite particles.
 18. A metal-air battery as recited in claim 9,wherein said electrocatalyst particles comprise a carbon support phaseand a platinum metal active species dispersed on said support phase. 19.A metal-air battery as recited in claim 9, wherein said electrocatalystcomprises MnO_(x) dispersed on a carbon support.
 20. A metal-air batteryas recited in claim 9, wherein said electrocatalyst comprises Co—Ni—O.21. A metal-air battery as recited in claim 9, wherein saidelectrocatalyst particles have an average size of not greater than about5 μm.
 22. A metal-air battery as recited in claim 9, wherein a majorityof said carbon particles in said polymer matrix are disposed near saidcurrent collector.
 23. A metal-air battery as recited in claim 9,wherein said anode is a zinc anode.
 24. A metal-air battery as recitedin claim 9, wherein said anode is a metal hydride anode.
 25. A metal-airbattery comprising an anode and a cathode, wherein said cathodecomprises: a) a gas diffusion layer; b) a current collector; and c) agradient functional layer, wherein said gradient functional layercomprises polymer-modified carbon particles having an average size ofnot greater than about 10 μm and electrocatalyst particles and whereinthe concentration of said carbon particles increases with increaseddistance from said anode, and wherein said current collector and saidfunctional gradient layer have an average thickness of not greater thanabout 100 μm.
 26. A metal-air battery as recited in claim 25, whereinsaid gas diffusion layer comprises a porous tetrafluoroethylenefluorocarbon polymer.
 27. A metal-air battery as recited in claim 25,wherein said current collector comprises elongate strips of a metal. 28.A metal-air battery as recited in claim 25, wherein said currentcollector comprises elongated strips of a metal having an average widthof not greater than about 100 μm.
 29. A metal-air battery as recited inclaim 25, wherein said carbon particles have an average particle size ofnot greater than about 10 μm.
 30. A metal-air battery as recited inclaim 25, wherein said gradient functional layer comprises atetrafluoroethylene fluorocarbon polymer.
 31. A metal-air battery asrecited in claim 25, wherein said functional gradient layer comprises ahydrophobicity gradient.
 32. A metal-air battery as recited in claim 25,wherein said electrocatalyst particles comprise carbon compositeparticles.
 33. A metal-air battery as recited in claim 25, wherein saidelectrocatalyst particles have an average particle size of not greaterthan about 10 μm.
 34. A metal-air battery as recited in claim 25,wherein said current collector and said functional gradient layer havean average thickness of not greater than about 50 μm.